diff --git a/nomenclature/cli.py b/nomenclature/cli.py
index 1a0f45e5..5e7a19aa 100644
--- a/nomenclature/cli.py
+++ b/nomenclature/cli.py
@@ -1,5 +1,4 @@
 from pathlib import Path
-from typing import List, Optional
 import importlib.util
 import sys
 
@@ -55,10 +54,10 @@ def cli_valid_yaml(path: Path):
 def cli_valid_project(
     path: Path,
     definitions: str,
-    mappings: Optional[str],
-    required_data: Optional[str],
-    validate_data: Optional[str],
-    dimensions: Optional[List[str]],
+    mappings: str | None,
+    required_data: str | None,
+    validate_data: str | None,
+    dimensions: list[str] | None,
 ):
     """Assert that `path` is a valid project nomenclature
 
@@ -74,7 +73,7 @@ def cli_valid_project(
         Name of folder for 'required data' criteria, default to "required_data"
     validate_data: str, optional
         Name of folder for data validation criteria, default to "validate_data"
-    dimensions : List[str], optional
+    dimensions : list[str], optional
         Dimensions to be checked, defaults to all sub-folders of `definitions`
 
     Example
@@ -125,8 +124,8 @@ def check_region_aggregation(
     workflow_directory: Path,
     definitions: str,
     mappings: str,
-    processed_data: Optional[Path],
-    differences: Optional[Path],
+    processed_data: Path | None,
+    differences: Path | None,
 ):
     """Perform region processing and compare aggregated and original data
 
@@ -141,10 +140,10 @@ def check_region_aggregation(
         Definitions folder inside workflow_directory, by default "definitions"
     mappings : str
         Model mapping folder inside workflow_directory, by default "mappings"
-    processed_data : Optional[Path]
+    processed_data : Path, optional
         If given, exports the results from region processing to a file called
         `processed_data`, by default "results.xlsx"
-    differences : Optional[Path]
+    differences : Path, optional
         If given, exports the differences between aggregated and model native data to a
         file called `differences`, by default None
 
@@ -295,7 +294,7 @@ def cli_run_workflow(
     multiple=True,
     default=None,
 )
-def cli_validate_scenarios(input_file: Path, definitions: Path, dimensions: List[str]):
+def cli_validate_scenarios(input_file: Path, definitions: Path, dimensions: list[str]):
     """Validate a scenario file against the codelists of a project
 
     Example
@@ -312,7 +311,7 @@ def cli_validate_scenarios(input_file: Path, definitions: Path, dimensions: List
         Input data file, must be IAMC format, .xlsx or .csv
     definitions : Path
         Definitions folder with codelists, by default "definitions"
-    dimensions : List[str], optional
+    dimensions : list[str], optional
         Dimensions to be checked, defaults to all sub-folders of `definitions`
 
     Raises
diff --git a/nomenclature/code.py b/nomenclature/code.py
index d92e3f9b..9c06bf36 100644
--- a/nomenclature/code.py
+++ b/nomenclature/code.py
@@ -2,7 +2,7 @@
 import re
 from keyword import iskeyword
 from pathlib import Path
-from typing import Any, Dict, List, Set, Union, Optional
+from typing import Any
 from pydantic import (
     field_validator,
     field_serializer,
@@ -24,8 +24,8 @@ class Code(BaseModel):
 
     name: str
     description: str | None = None
-    file: Union[str, Path] | None = None
-    extra_attributes: Dict[str, Any] = {}
+    file: str | Path | None = None
+    extra_attributes: dict[str, Any] = {}
     repository: str | None = None
 
     def __eq__(self, other) -> bool:
@@ -34,8 +34,8 @@ def __eq__(self, other) -> bool:
     @field_validator("extra_attributes")
     @classmethod
     def check_attribute_names(
-        cls, v: Dict[str, Any], info: ValidationInfo
-    ) -> Dict[str, Any]:
+        cls, v: dict[str, Any], info: ValidationInfo
+    ) -> dict[str, Any]:
         # Check that attributes only contains keys which are valid identifiers
         if illegal_keys := [
             key for key in v.keys() if not key.isidentifier() or iskeyword(key)
@@ -79,7 +79,7 @@ def from_dict(cls, mapping) -> "Code":
         )
 
     @classmethod
-    def named_attributes(cls) -> Set[str]:
+    def named_attributes(cls) -> set[str]:
         return {a for a in cls.model_fields if a != "extra_attributes"}
 
     @property
@@ -181,10 +181,10 @@ def __setattr__(self, name, value):
 
 
 class VariableCode(Code):
-    unit: Union[str, List[str]] = Field(...)
+    unit: str | list[str] = Field(...)
     tier: int | str | None = None
     weight: str | None = None
-    region_aggregation: List[Dict[str, Dict]] | None = Field(
+    region_aggregation: list[dict[str, dict]] | None = Field(
         default=None, alias="region-aggregation"
     )
     skip_region_aggregation: bool | None = Field(
@@ -192,7 +192,7 @@ class VariableCode(Code):
     )
     method: str | None = None
     check_aggregate: bool | None = Field(default=False, alias="check-aggregate")
-    components: Union[List[str], Dict[str, list[str]]] | None = None
+    components: list[str] | dict[str, list[str]] | None = None
     drop_negative_weights: bool | None = None
     model_config = ConfigDict(populate_by_name=True)
 
@@ -225,17 +225,17 @@ def convert_str_to_none_for_writing(self, v):
         return v if v != "" else None
 
     @property
-    def units(self) -> List[str]:
+    def units(self) -> list[str]:
         return self.unit if isinstance(self.unit, list) else [self.unit]
 
     @classmethod
-    def named_attributes(cls) -> Set[str]:
+    def named_attributes(cls) -> set[str]:
         return (
             super().named_attributes().union(f.alias for f in cls.model_fields.values())
         )
 
     @property
-    def pyam_agg_kwargs(self) -> Dict[str, Any]:
+    def pyam_agg_kwargs(self) -> dict[str, Any]:
         # return a dict of all not None pyam aggregation properties
         return {
             field: getattr(self, field)
@@ -249,7 +249,7 @@ def pyam_agg_kwargs(self) -> Dict[str, Any]:
         }
 
     @property
-    def agg_kwargs(self) -> Dict[str, Any]:
+    def agg_kwargs(self) -> dict[str, Any]:
         return (
             {**self.pyam_agg_kwargs, **{"region_aggregation": self.region_aggregation}}
             if self.region_aggregation is not None
@@ -274,11 +274,11 @@ class RegionCode(Code):
     """
 
     hierarchy: str = None
-    countries: Optional[List[str]] = None
-    iso3_codes: Optional[Union[List[str], str]] = None
+    countries: list[str] | None = None
+    iso3_codes: list[str] | str | None = None
 
     @field_validator("countries", mode="before")
-    def check_countries(cls, v: List[str], info: ValidationInfo) -> List[str]:
+    def check_countries(cls, v: list[str], info: ValidationInfo) -> list[str]:
         """Verifies that each country name is defined in `nomenclature.countries`."""
         v = to_list(v)
         if invalid_country_names := set(v) - set(countries.names):
@@ -291,7 +291,7 @@ def check_countries(cls, v: List[str], info: ValidationInfo) -> List[str]:
         return v
 
     @field_validator("iso3_codes")
-    def check_iso3_codes(cls, v: List[str], info: ValidationInfo) -> List[str]:
+    def check_iso3_codes(cls, v: list[str], info: ValidationInfo) -> list[str]:
         """Verifies that each ISO3 code is valid according to pycountry library."""
         errors = ErrorCollector()
         if invalid_iso3_codes := [
@@ -315,9 +315,9 @@ class MetaCode(Code):
 
     Attributes
     ----------
-    allowed_values : Optional(list[any])
+    allowed_values : list[Any], optional
         An optional list of allowed values
 
     """
 
-    allowed_values: List[Any] | None = None
+    allowed_values: list[Any] | None = None
diff --git a/nomenclature/codelist.py b/nomenclature/codelist.py
index 1c177a0c..16adaa26 100644
--- a/nomenclature/codelist.py
+++ b/nomenclature/codelist.py
@@ -1,7 +1,7 @@
 import logging
 from pathlib import Path
 from textwrap import indent
-from typing import ClassVar, Dict, List
+from typing import ClassVar
 
 import numpy as np
 import pandas as pd
@@ -34,7 +34,7 @@ class CodeList(BaseModel):
     """
 
     name: str
-    mapping: Dict[str, Code] = {}
+    mapping: dict[str, Code] = {}
 
     # class variable
     validation_schema: ClassVar[str] = "generic"
@@ -46,8 +46,8 @@ def __eq__(self, other):
     @field_validator("mapping")
     @classmethod
     def check_stray_tag(
-        cls, v: Dict[str, Code], info: ValidationInfo
-    ) -> Dict[str, Code]:
+        cls, v: dict[str, Code], info: ValidationInfo
+    ) -> dict[str, Code]:
         """Check that no stray tags are left in codes after tag replacement"""
         forbidden = ["{", "}"]
 
@@ -75,8 +75,8 @@ def _check_string(value):
     @field_validator("mapping")
     @classmethod
     def check_end_whitespace(
-        cls, v: Dict[str, Code], info: ValidationInfo
-    ) -> Dict[str, Code]:
+        cls, v: dict[str, Code], info: ValidationInfo
+    ) -> dict[str, Code]:
         """Check that no code ends with a whitespace"""
         for code in v:
             if code.endswith(" "):
@@ -127,7 +127,7 @@ def validate_data(
             return False
         return True
 
-    def validate_items(self, items: List[str]) -> List[str]:
+    def validate_items(self, items: list[str]) -> list[str]:
         """Validate that a list of items are valid codes
 
         Returns
@@ -140,9 +140,9 @@ def validate_items(self, items: List[str]) -> List[str]:
 
     @classmethod
     def replace_tags(
-        cls, code_list: List[Code], tag_name: str, tags: List[Code]
-    ) -> List[Code]:
-        _code_list: List[Code] = []
+        cls, code_list: list[Code], tag_name: str, tags: list[Code]
+    ) -> list[Code]:
+        _code_list: list[Code] = []
 
         for code in code_list:
             if "{" + tag_name + "}" in code.name:
@@ -155,15 +155,15 @@ def replace_tags(
     @classmethod
     def _parse_and_replace_tags(
         cls,
-        code_list: List[Code],
+        code_list: list[Code],
         path: Path,
         file_glob_pattern: str = "**/*",
-    ) -> List[Code]:
+    ) -> list[Code]:
         """Cast, validate and replace tags into list of codes for one dimension
 
         Parameters
         ----------
-        code_list : List[Code]
+        code_list : list[Code]
             List of Code to modify
         path : :class:`pathlib.Path` or path-like
             Directory with the codelist files
@@ -173,10 +173,10 @@ def _parse_and_replace_tags(
 
         Returns
         -------
-        Dict[str, Code] :class: `nomenclature.Code`
+        dict[str, Code] :class: `nomenclature.Code`
 
         """
-        tag_dict: Dict[str, List[Code]] = {}
+        tag_dict: dict[str, list[Code]] = {}
 
         for yaml_file in (
             f
@@ -240,7 +240,7 @@ def from_directory(
             )
             code_list.extend(repo.filter_list_of_codes(repository_code_list))
         errors = ErrorCollector()
-        mapping: Dict[str, Code] = {}
+        mapping: dict[str, Code] = {}
         for code in code_list:
             if code.name in mapping:
                 errors.append(
@@ -301,7 +301,7 @@ def _parse_codelist_dir(
         file_glob_pattern: str = "**/*",
         repository: str | None = None,
     ):
-        code_list: List[Code] = []
+        code_list: list[Code] = []
         for yaml_file in (
             f
             for f in path.glob(file_glob_pattern)
@@ -457,7 +457,7 @@ def to_excel(
             with pd.ExcelWriter(excel_writer, **kwargs) as writer:
                 write_sheet(writer, sheet_name, self.to_pandas(sort_by_code))
 
-    def codelist_repr(self, json_serialized=False) -> Dict:
+    def codelist_repr(self, json_serialized=False) -> dict:
         """Cast a CodeList into corresponding dictionary"""
 
         nice_dict = {}
@@ -590,7 +590,7 @@ def check_weight_in_vars(cls, v):
             )
         return v
 
-    def vars_default_args(self, variables: List[str]) -> List[VariableCode]:
+    def vars_default_args(self, variables: list[str]) -> list[VariableCode]:
         """return subset of variables which does not feature any special pyam
         aggregation arguments and where skip_region_aggregation is False"""
         return [
@@ -599,7 +599,7 @@ def vars_default_args(self, variables: List[str]) -> List[VariableCode]:
             if not self[var].agg_kwargs and not self[var].skip_region_aggregation
         ]
 
-    def vars_kwargs(self, variables: List[str]) -> List[VariableCode]:
+    def vars_kwargs(self, variables: list[str]) -> list[VariableCode]:
         # return subset of variables which features special pyam aggregation arguments
         # and where skip_region_aggregation is False
         return [
@@ -713,7 +713,7 @@ def from_directory(
 
         """
 
-        code_list: List[RegionCode] = []
+        code_list: list[RegionCode] = []
 
         # initializing from general configuration
         # adding all countries
@@ -763,7 +763,7 @@ def from_directory(
         )
 
         # translate to mapping
-        mapping: Dict[str, RegionCode] = {}
+        mapping: dict[str, RegionCode] = {}
 
         errors = ErrorCollector()
         for code in code_list:
@@ -783,12 +783,12 @@ def from_directory(
         return cls(name=name, mapping=mapping)
 
     @property
-    def hierarchy(self) -> List[str]:
+    def hierarchy(self) -> list[str]:
         """Return the hierarchies defined in the RegionCodeList
 
         Returns
         -------
-        List[str]
+        list[str]
 
         """
         return sorted(list({v.hierarchy for v in self.mapping.values()}))
@@ -799,9 +799,9 @@ def _parse_region_code_dir(
         path: Path,
         file_glob_pattern: str = "**/*",
         repository: str | None = None,
-    ) -> List[RegionCode]:
+    ) -> list[RegionCode]:
         """"""
-        code_list: List[RegionCode] = []
+        code_list: list[RegionCode] = []
         for yaml_file in (
             f
             for f in path.glob(file_glob_pattern)
diff --git a/nomenclature/core.py b/nomenclature/core.py
index 4da95f3d..8e38c547 100644
--- a/nomenclature/core.py
+++ b/nomenclature/core.py
@@ -1,5 +1,4 @@
 import logging
-from typing import Optional, Union, List
 
 import pyam
 from pydantic import validate_call
@@ -14,8 +13,8 @@
 def process(
     df: pyam.IamDataFrame,
     dsd: DataStructureDefinition,
-    dimensions: Optional[List[str]] = None,
-    processor: Optional[Union[Processor, List[Processor]]] = None,
+    dimensions: list[str] | None = None,
+    processor: Processor | list[Processor] | None = None,
 ) -> pyam.IamDataFrame:
     """Function for validation and region aggregation in one step
 
diff --git a/nomenclature/error.py b/nomenclature/error.py
index 6549a908..4cdca95f 100644
--- a/nomenclature/error.py
+++ b/nomenclature/error.py
@@ -1,7 +1,6 @@
 import logging
 import textwrap
 from collections import namedtuple
-from typing import Optional
 
 pydantic_custom_error_config = {
     "RegionNameCollisionError": (
@@ -49,7 +48,7 @@
 
 class ErrorCollector:
     errors: list[Exception]
-    description: Optional[str] = None
+    description: str | None = None
 
     def __init__(self, description: str = None) -> None:
         self.errors = []
diff --git a/nomenclature/processor/data_validator.py b/nomenclature/processor/data_validator.py
index c0731aac..6bd38df5 100644
--- a/nomenclature/processor/data_validator.py
+++ b/nomenclature/processor/data_validator.py
@@ -1,7 +1,6 @@
 import logging
 import textwrap
 from pathlib import Path
-from typing import List, Optional, Union
 
 import yaml
 from pyam import IamDataFrame
@@ -52,8 +51,8 @@ def criteria(self):
 
 
 class DataValidationCriteriaBounds(IamcDataFilter):
-    upper_bound: Optional[float] = None
-    lower_bound: Optional[float] = None
+    upper_bound: float | None = None
+    lower_bound: float | None = None
 
     @model_validator(mode="after")
     def check_validation_criteria_exist(self):
@@ -69,7 +68,7 @@ def validation_args(self):
 class DataValidator(Processor):
     """Processor for validating IAMC datapoints"""
 
-    criteria_items: List[DataValidationCriteriaBounds | DataValidationCriteriaValue]
+    criteria_items: list[DataValidationCriteriaBounds | DataValidationCriteriaValue]
     file: Path
 
     @field_validator("criteria_items", mode="before")
@@ -84,7 +83,7 @@ def check_criteria(cls, v):
         return v
 
     @classmethod
-    def from_file(cls, file: Union[Path, str]) -> "DataValidator":
+    def from_file(cls, file: Path | str) -> "DataValidator":
         with open(file, "r", encoding="utf-8") as f:
             content = yaml.safe_load(f)
         return cls(file=file, criteria_items=content)
diff --git a/nomenclature/processor/iamc.py b/nomenclature/processor/iamc.py
index 1eba7abc..3cba8d98 100644
--- a/nomenclature/processor/iamc.py
+++ b/nomenclature/processor/iamc.py
@@ -1,4 +1,3 @@
-from typing import List
 from pydantic import BaseModel, ConfigDict, field_validator
 
 from pyam import IAMC_IDX
@@ -9,12 +8,12 @@
 class IamcDataFilter(BaseModel):
     model_config = ConfigDict(extra="forbid")
 
-    model: List[str] | None = None
-    scenario: List[str] | None = None
-    region: List[str] | None = None
-    variable: List[str] | None = None
-    unit: List[str] | None = None
-    year: List[int] | None = None
+    model: list[str] | None = None
+    scenario: list[str] | None = None
+    region: list[str] | None = None
+    variable: list[str] | None = None
+    unit: list[str] | None = None
+    year: list[int] | None = None
 
     @field_validator(*IAMC_IDX + ["year"], mode="before")
     @classmethod
diff --git a/nomenclature/processor/region.py b/nomenclature/processor/region.py
index 92ac5248..06b56ec9 100644
--- a/nomenclature/processor/region.py
+++ b/nomenclature/processor/region.py
@@ -1,7 +1,6 @@
 import logging
 from collections import Counter
 from pathlib import Path
-from typing import Dict, List, Optional, Tuple, Union
 from typing_extensions import Annotated
 
 import numpy as np
@@ -42,12 +41,12 @@ class NativeRegion(BaseModel):
     ----------
     name : str
         Name of the model native region.
-    rename: Optional[str]
+    rename: str, optional
         Optional second name that the region will be renamed to.
     """
 
     name: str
-    rename: Optional[str] = None
+    rename: str | None = None
 
     @property
     def target_native_region(self) -> str:
@@ -74,7 +73,7 @@ class CommonRegion(BaseModel):
     """
 
     name: str
-    constituent_regions: List[str]
+    constituent_regions: list[str]
 
     @property
     def is_single_constituent_region(self):
@@ -102,17 +101,17 @@ class RegionAggregationMapping(BaseModel):
         Name of the model for which RegionAggregationMapping is defined.
     file: FilePath
         File path of the mapping file. Saved mostly for error reporting purposes.
-    native_regions: Optional[List[NativeRegion]]
+    native_regions: list[NativeRegion], optional
         Optionally, list of model native regions to select and potentially rename.
-    common_regions: Optional[List[CommonRegion]]
+    common_regions: list[CommonRegion], optional
         Optionally, list of common regions where aggregation will be performed.
     """
 
-    model: List[str]
+    model: list[str]
     file: FilePath
-    native_regions: List[NativeRegion] | None = None
-    common_regions: List[CommonRegion] | None = None
-    exclude_regions: List[str] | None = None
+    native_regions: list[NativeRegion] | None = None
+    common_regions: list[CommonRegion] | None = None
+    exclude_regions: list[str] | None = None
 
     @model_validator(mode="before")
     @classmethod
@@ -234,12 +233,12 @@ def check_exclude_common_region_overlap(
         return _check_exclude_region_overlap(v, "common_regions")
 
     @classmethod
-    def from_file(cls, file: Union[Path, str]):
+    def from_file(cls, file: Path | str):
         """Initialize a RegionAggregationMapping from a file.
 
         Parameters
         ----------
-        file : Union[Path, str]
+        file : Path | str
             Path to a yaml file which contains region aggregation information for one
             model.
 
@@ -276,7 +275,7 @@ def from_yaml(cls, file: Path) -> "RegionAggregationMapping":
 
             # Reformat the "native_regions"
             if "native_regions" in mapping_input:
-                native_region_list: List[Dict] = []
+                native_region_list: list[dict] = []
                 for native_region in mapping_input["native_regions"]:
                     if isinstance(native_region, str):
                         native_region_list.append({"name": native_region})
@@ -291,7 +290,7 @@ def from_yaml(cls, file: Path) -> "RegionAggregationMapping":
 
             # Reformat the "common_regions"
             if "common_regions" in mapping_input:
-                common_region_list: List[Dict[str, List[Dict[str, str]]]] = []
+                common_region_list: list[dict[str, list[dict[str, str]]]] = []
                 for common_region in mapping_input["common_regions"]:
                     common_region_name = list(common_region)[0]
                     common_region_list.append(
@@ -351,34 +350,34 @@ def from_excel(cls, file) -> "RegionAggregationMapping":
         )
 
     @property
-    def all_regions(self) -> List[str]:
+    def all_regions(self) -> list[str]:
         # For the native regions we take the **renamed** (if given) names
         nr_list = [x.target_native_region for x in self.native_regions or []]
         return nr_list + self.common_region_names
 
     @property
-    def model_native_region_names(self) -> List[str]:
+    def model_native_region_names(self) -> list[str]:
         # List of the **original** model native region names
         return [x.name for x in self.native_regions or []]
 
     @property
-    def common_region_names(self) -> List[str]:
+    def common_region_names(self) -> list[str]:
         # List of the common region names
         return [x.name for x in self.common_regions or []]
 
     @property
-    def rename_mapping(self) -> Dict[str, str]:
+    def rename_mapping(self) -> dict[str, str]:
         return {r.name: r.target_native_region for r in self.native_regions or []}
 
     @property
-    def upload_native_regions(self) -> List[str]:
+    def upload_native_regions(self) -> list[str]:
         return [
             native_region.target_native_region
             for native_region in self.native_regions or []
         ]
 
     @property
-    def reverse_rename_mapping(self) -> Dict[str, str]:
+    def reverse_rename_mapping(self) -> dict[str, str]:
         return {renamed: original for original, renamed in self.rename_mapping.items()}
 
     def check_unexpected_regions(self, df: IamDataFrame) -> None:
@@ -444,7 +443,7 @@ class RegionProcessor(Processor):
 
     region_codelist: RegionCodeList
     variable_codelist: VariableCodeList
-    mappings: Dict[
+    mappings: dict[
         str,
         Annotated[RegionAggregationMapping, AfterValidator(validate_with_definition)],
     ]
@@ -477,7 +476,7 @@ def from_directory(cls, path: DirectoryPath, dsd: DataStructureDefinition):
             Raised if the provided DataStructureDefinition does not contain the dimensions ``region`` and ``variable``.
 
         """
-        mapping_dict: Dict[str, RegionAggregationMapping] = {}
+        mapping_dict: dict[str, RegionAggregationMapping] = {}
         errors = ErrorCollector()
 
         mapping_files = [f for f in path.glob("**/*") if f.suffix in {".yaml", ".yml"}]
@@ -543,7 +542,7 @@ def apply(self, df: IamDataFrame) -> IamDataFrame:
             * If *df* contains regions that are not listed in the model mapping, or
             * If the region-processing results in an empty **IamDataFrame**.
         """
-        processed_dfs: List[IamDataFrame] = []
+        processed_dfs: list[IamDataFrame] = []
 
         for model in df.model:
             model_df = df.filter(model=model)
@@ -571,7 +570,7 @@ def apply(self, df: IamDataFrame) -> IamDataFrame:
 
     def check_region_aggregation(
         self, df: IamDataFrame, rtol_difference: float = 0.01
-    ) -> Tuple[IamDataFrame, pd.DataFrame]:
+    ) -> tuple[IamDataFrame, pd.DataFrame]:
         """Return region aggregation results and differences between aggregated and
         model native data
 
@@ -584,7 +583,7 @@ def check_region_aggregation(
 
         Returns
         -------
-        Tuple[IamDataFrame, pd.DataFrame]
+        tuple[IamDataFrame, pd.DataFrame]
             IamDataFrame containing aggregation results and pandas dataframe containing
             the differences
         """
@@ -605,7 +604,7 @@ def _apply_region_processing(
         model_df: IamDataFrame,
         return_aggregation_difference: bool = False,
         rtol_difference: float = 0.01,
-    ) -> Tuple[IamDataFrame, pd.DataFrame]:
+    ) -> tuple[IamDataFrame, pd.DataFrame]:
         """Apply the region processing for a single model"""
         if len(model_df.model) != 1:
             raise ValueError(
@@ -616,7 +615,7 @@ def _apply_region_processing(
         # check for regions not mentioned in the model mapping
         self.mappings[model].check_unexpected_regions(model_df)
 
-        _processed_data: List[pd.Series] = []
+        _processed_data: list[pd.Series] = []
 
         # silence pyam's empty filter warnings
         with adjust_log_level(logger="pyam", level="ERROR"):
@@ -747,7 +746,7 @@ def _compare_and_merge(
     aggregated: pd.Series,
     rtol: float = 0.01,
     return_aggregation_difference: bool = False,
-) -> Tuple[IamDataFrame, pd.DataFrame]:
+) -> tuple[IamDataFrame, pd.DataFrame]:
     """Compare and merge original and aggregated results"""
 
     # compare processed (aggregated) data and data provided at the common-region level
diff --git a/nomenclature/processor/required_data.py b/nomenclature/processor/required_data.py
index 76d124c2..68826085 100644
--- a/nomenclature/processor/required_data.py
+++ b/nomenclature/processor/required_data.py
@@ -1,6 +1,6 @@
 import logging
 from pathlib import Path
-from typing import Any, List, Tuple, Union, Annotated
+from typing import Any, Annotated
 
 import pandas as pd
 import yaml
@@ -24,7 +24,7 @@
 
 class RequiredMeasurand(BaseModel):
     variable: str
-    unit: List[Union[str, None]] = Field(...)
+    unit: list[str | None] = Field(...)
 
     @field_validator("unit", mode="before")
     @classmethod
@@ -43,12 +43,12 @@ def cast_to_RequiredMeasurand(v) -> RequiredMeasurand:
 
 class RequiredData(BaseModel):
     measurand: (
-        List[Annotated[RequiredMeasurand, BeforeValidator(cast_to_RequiredMeasurand)]]
+        list[Annotated[RequiredMeasurand, BeforeValidator(cast_to_RequiredMeasurand)]]
         | None
     ) = None
-    variable: List[str] | None = None
-    region: List[str] | None = None
-    year: List[int] | None = None
+    variable: list[str] | None = None
+    region: list[str] | None = None
+    year: list[int] | None = None
     # TODO consider merging with IamcDataFilter
 
     @field_validator("measurand", "region", "year", "variable", mode="before")
@@ -99,13 +99,13 @@ def validate_with_definition(self, dsd: DataStructureDefinition) -> None:
             raise ValueError(error_msg)
 
     @property
-    def variables(self) -> List[str]:
+    def variables(self) -> list[str]:
         if self.measurand is not None:
             return [m.variable for m in self.measurand]
         return self.variable
 
     @property
-    def pyam_required_data_list(self) -> List[List[dict]]:
+    def pyam_required_data_list(self) -> list[list[dict]]:
         if self.measurand is not None:
             return [
                 [
@@ -132,8 +132,8 @@ def pyam_required_data_list(self) -> List[List[dict]]:
 
     def _wrong_unit_variables(
         self, dsd: DataStructureDefinition
-    ) -> List[Tuple[str, str, str]]:
-        wrong_units: List[Tuple[str, Any, Any]] = []
+    ) -> list[tuple[str, str, str]]:
+        wrong_units: list[tuple[str, Any, Any]] = []
         if hasattr(dsd, "variable") and self.measurand is not None:
             wrong_units.extend(
                 (m.variable, unit, dsd.variable[m.variable].unit)
@@ -148,8 +148,8 @@ def _wrong_unit_variables(
 
 class RequiredDataValidator(Processor):
     description: str | None = None
-    model: List[str] | None = None
-    required_data: List[RequiredData]
+    model: list[str] | None = None
+    required_data: list[RequiredData]
     file: Path
 
     @field_validator("model", mode="before")
@@ -158,7 +158,7 @@ def convert_to_list(cls, v):
         return pyam.utils.to_list(v)
 
     @classmethod
-    def from_file(cls, file: Union[Path, str]) -> "RequiredDataValidator":
+    def from_file(cls, file: Path | str) -> "RequiredDataValidator":
         with open(file, "r", encoding="utf-8") as f:
             content = yaml.safe_load(f)
         return cls(file=file, **content)
@@ -195,7 +195,7 @@ def apply(self, df: IamDataFrame) -> IamDataFrame:
 
     def check_required_data_per_model(
         self, df: IamDataFrame, model: str
-    ) -> List[pyam.IamDataFrame]:
+    ) -> list[pyam.IamDataFrame]:
         model_df = df.filter(model=model)
         missing_data = []
         for requirement in self.required_data:
diff --git a/nomenclature/testing.py b/nomenclature/testing.py
index 4cc524de..97a052a9 100644
--- a/nomenclature/testing.py
+++ b/nomenclature/testing.py
@@ -1,6 +1,5 @@
 import logging
 from pathlib import Path
-from typing import List, Optional
 
 import yaml
 
@@ -55,7 +54,7 @@ def assert_valid_yaml(path: Path):
 def _check_mappings(
     path: Path,
     dsd: DataStructureDefinition,
-    mappings: Optional[str] = None,
+    mappings: str | None = None,
 ) -> None:
     if mappings is None:
         if (path / "mappings").is_dir():
@@ -86,7 +85,7 @@ def _check_processor_directory(
     dsd: DataStructureDefinition,
     processor: Processor,
     processor_arg: str,
-    folder: Optional[str] = None,
+    folder: str | None = None,
 ) -> None:
     if folder is None:
         if (path / processor_arg).is_dir():
@@ -102,10 +101,10 @@ def _check_processor_directory(
 def assert_valid_structure(
     path: Path,
     definitions: str = "definitions",
-    mappings: Optional[str] = None,
-    required_data: Optional[str] = None,
-    validate_data: Optional[str] = None,
-    dimensions: Optional[List[str]] = None,
+    mappings: str | None = None,
+    required_data: str | None = None,
+    validate_data: str | None = None,
+    dimensions: list[str] | None = None,
 ) -> None:
     """Assert that `path` can be initialized as a :class:`DataStructureDefinition`
 
@@ -123,7 +122,7 @@ def assert_valid_structure(
     validate_data : str, optional
         Name of the folder for data validation criteria, defaults to "validate_date"
         (if this folder exists)
-    dimensions : List[str], optional
+    dimensions : list[str], optional
         Dimensions to be checked, defaults to all sub-folders of `definitions`
 
     Notes
diff --git a/tests/test_codelist.py b/tests/test_codelist.py
index 6efa7f99..82d8564d 100644
--- a/tests/test_codelist.py
+++ b/tests/test_codelist.py
@@ -358,7 +358,8 @@ def test_RegionCodeList_filter():
 
 
 def test_RegionCodeList_hierarchy():
-    """Verifies that the hierarchy method returns a List[str]"""
+    """Verifies that the hierarchy method returns a list"""
+
 
     rcl = RegionCodeList.from_directory(
         "Region", MODULE_TEST_DATA_DIR / "region_to_filter_codelist"