diff --git a/README.md b/README.md
index fd24553..7dbbb52 100644
--- a/README.md
+++ b/README.md
@@ -103,11 +103,59 @@ In some cases, it is desired to stop the engine as soon as a specific rule has f
 This is achieved by settings the respective rules' property `final` to `true`.
 Default, of course, is `false`.
 
+### Optimization of premises (`when`)
+
+It is very common that different rules partially share the same premises.
+Rools will merge identical premises into one.
+You are free to use references or just to repeat the same premise.
+Both cases are working fine.
+
+Example 1: by reference
+```js
+const isApplicable = facts => facts.user.salery >= 2000;
+const rule1 = {
+  when: isApplicable,
+  ...
+};
+const rule2 = {
+  when: isApplicable,
+  ...
+};
+```
+
+Example 2: repeat premise
+```js
+const rule1 = {
+  when: facts => facts.user.salery >= 2000,
+  ...
+};
+const rule2 = {
+  when: facts => facts.user.salery >= 2000,
+  ...
+};
+```
+
+TL;DR
+
+Technically, this is achieved by hashing the premises
+(remember, a function is an object in JavaScript).
+This can be a classic function or an ES6 arrow function.
+This can be a reference or the function directly.
+It's tested with Node 8 and 9 (see unit tests `premises.spec.js`).
+
+```
+const md5 = require('md5');
+const hash1 = md5(facts => facts.user.salery > 2000);
+const hash2 = md5(facts => facts.user.salery > 2000);
+const hash3 = md5(facts => facts.user.salery > 3000);
+console.log(hash1 === hash2); // true
+console.log(hash1 === hash3); // false
+```
+
 ### Todos
 
 Some of the features on my list are:
  * Conflict resolution by specificity
- * Optimization: merge identical premises (`when`) into one
  * Optimization: re-evaluate only those premises (`when`) that are relying on modified facts
  * Support asynchronous actions (`then`)
  * More unit tests
diff --git a/src/index.js b/src/index.js
index 58c4ced..30c9d81 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,4 +1,5 @@
 const assert = require('assert');
+const md5 = require('md5');
 const actionId = require('uniqueid')('a');
 const premiseId = require('uniqueid')('p');
 
@@ -8,6 +9,7 @@ class Rools {
   } = { logErrors: true, logDebug: false, logDelegate: null }) {
     this.actions = [];
     this.premises = [];
+    this.premisesByHash = {};
     this.maxSteps = 100;
     this.logErrors = logErrors;
     this.logDebug = logDebug;
@@ -30,13 +32,18 @@ class Rools {
       this.actions.push(action);
       const whens = Array.isArray(rule.when) ? rule.when : [rule.when];
       whens.forEach((when) => {
-        const premise = {
-          id: premiseId(),
-          name: rule.name,
-          when,
-        };
-        action.premises.push(premise);
-        this.premises.push(premise);
+        const hash = md5(when); // is function already introduced by other rule?
+        let premise = this.premisesByHash[hash];
+        if (!premise) { // create new premise
+          premise = {
+            id: premiseId(),
+            name: rule.name,
+            when,
+          };
+          this.premisesByHash[hash] = premise; // add to hash
+          this.premises.push(premise); // add to premises
+        }
+        action.premises.push(premise); // add to action
       });
     });
   }
diff --git a/test/premises.spec.js b/test/premises.spec.js
new file mode 100644
index 0000000..969b34d
--- /dev/null
+++ b/test/premises.spec.js
@@ -0,0 +1,125 @@
+const Rools = require('../src');
+require('./setup');
+
+describe('Rules.register() / optimization of premises', () => {
+  it('should not merge premises if not identical', () => {
+    const rule1 = {
+      name: 'rule1',
+      when: facts => facts.user.name === 'frank',
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: facts => facts.user.name === 'michael',
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(2);
+  });
+
+  it('should merge premises if identical / reference / arrow function', () => {
+    const isFrank = facts => facts.user.name === 'frank';
+    const rule1 = {
+      name: 'rule1',
+      when: isFrank,
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: isFrank,
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(1);
+  });
+
+  it('should merge premises if identical / reference / classic function', () => {
+    function isFrank(facts) {
+      return facts.user.name === 'frank';
+    }
+    const rule1 = {
+      name: 'rule1',
+      when: isFrank,
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: isFrank,
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(1);
+  });
+
+  it('should merge premises if identical / hash / arrow function', () => {
+    const rule1 = {
+      name: 'rule1',
+      when: facts => facts.user.name === 'frank',
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: facts => facts.user.name === 'frank',
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(1);
+  });
+
+  it('should merge premises if identical / hash / classic function()', () => {
+    const rule1 = {
+      name: 'rule1',
+      when: function p(facts) {
+        return facts.user.name === 'frank';
+      },
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: function p(facts) {
+        return facts.user.name === 'frank';
+      },
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(1);
+  });
+
+  it('should merge premises if identical / hash / slightly different', () => {
+    const rule1 = {
+      name: 'rule1',
+      when: facts => facts.user.name === 'frank',
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: facts => facts.user.name === "frank", // eslint-disable-line quotes
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(1);
+  });
+
+  it('should merge premises if identical / with Date object', () => {
+    const date = new Date('2000-01-01');
+    const rule1 = {
+      name: 'rule1',
+      when: facts => facts.user.birthdate > date,
+      then: () => {},
+    };
+    const rule2 = {
+      name: 'rule2',
+      when: facts => facts.user.birthdate > date,
+      then: () => {},
+    };
+    const rools = new Rools();
+    rools.register(rule1, rule2);
+    expect(rools.premises.length).to.be.equal(1);
+  });
+});