diff --git a/README.md b/README.md
index 64b9b02..af2022c 100644
--- a/README.md
+++ b/README.md
@@ -16,8 +16,6 @@ Rules are specified in pure JavaScript rather than in a separate, special-purpos
 
 *Secondary design goal* was to provide RETE-like efficiency and optimization.
 
-It uses some of the cool new ES6 stuff, e.g., Generators, `Proxy`, `Reflect`, `Set`. *JavaScript Rocks!*
-
 ## Install
 
 ```bash
@@ -67,7 +65,7 @@ const Rools = require('rools');
 
 const rools = new Rools();
 rools.register(ruleMoodGreat, ruleGoWalking);
-const result = rools.evaluate(facts);
+const result = await rools.evaluate(facts);
 ```
 This is the result:
 ```js
@@ -83,8 +81,6 @@ This is the result:
 
 The engine does forward-chaining and works in the usual match-resolve-act cycle.
 
-Rule evaluation is non-blocking, i.e., each evaluation step is one execution block (using ES6 Generators).
-
 ### Conflict resolution
 
 If there is more than one rule ready to fire, i.e., the conflict set is greater 1, the following conflict resolution strategies are applied (in this order):
@@ -98,6 +94,33 @@ For optimization purposes, it might be desired to stop the engine as soon as a s
 This can be achieved by settings the respective rules' property `final` to `true`.
 Default, of course, is `false`.
 
+### Async actions
+
+While premises (`when`) are always working synchronously on the facts,
+actions (`then`) can be synchronous or asynchronous.
+
+Example: asynchronous action using async/await
+```js
+const rule = {
+  name: 'check availability',
+  when: facts => facts.user.address.country === 'germany',
+  then: async (facts) => {
+    facts.products = await availabilityCheck(facts.user.address);
+  },
+};
+```
+
+Example: synchronous action using promises
+```js
+const rule = {
+  name: 'check availability',
+  when: facts => facts.user.address.country === 'germany',
+  then: facts =>
+    availabilityCheck(facts.user.address)
+      .then((result) => { facts.products = result; }),
+};
+```
+
 ### Optimization I
 
 It is very common that different rules partially share the same premises.
@@ -187,7 +210,7 @@ const facts = {
   ...
 };
 ...
-rools.evaluate(facts);
+await rools.evaluate(facts);
 ```
 
 This optimization targets runtime performance.
@@ -216,8 +239,8 @@ Rules are plain JavaScript objects with the following properties:
 | Property    | Required | Default | Description |
 |-------------|----------|---------|-------------|
 | `name`      | yes      | -       | A string value identifying the rule. This is used logging and debugging purposes only. |
-| `when`      | yes      | -       | A JavaScript function or an array of functions. These are the premises of your rule. The functions' interface is `(facts) => ...`. They must return a boolean value. |
-| `then`      | yes      | -       | A JavaScript function to be executed when the rule fires. The function's interface is `(facts) => { ... }`. |
+| `when`      | yes      | -       | A synchronous JavaScript function or an array of functions. These are the premises of your rule. The functions' interface is `(facts) => ...`. They must return a boolean value. |
+| `then`      | yes      | -       | A synchronous or asynchronous JavaScript function to be executed when the rule fires. The function's interface is `(facts) => { ... }` or `async (facts) => { ... }`. |
 | `priority`  | no       | `0`     | If during `evaluate()` there is more than one rule ready to fire, i.e., the conflict set is greater 1, rules with higher priority will fire first. Negative values are supported. |
 | `final`     | no       | `false` | Marks a rule as final. If during `evaluate()` a final rule fires, the engine will stop the evaluation. |
 
@@ -225,7 +248,7 @@ Rules are plain JavaScript objects with the following properties:
 It can be called multiple time.
 New rules will become effective immediately.
 
-`register()` may `throw` an exception, e.g., if a rule is formally incorrect.
+`register()` is working synchronously and may `throw` an exception, e.g., if a rule is formally incorrect.
 If an exception is thrown, the affected Rools instance becomes inconsistent and should no longer be used.
 
 Example:
@@ -252,7 +275,7 @@ const rools = new Rools();
 rools.register(ruleMoodGreat, ruleGoWalking);
 ```
 
-### `evaluate()` -- evaluate facts
+### `async evaluate()` -- evaluate facts
 
 Facts are plain JavaScript or JSON objects. For example:
 ```js
@@ -269,7 +292,7 @@ const facts = {
 };
 const rools = new Rools();
 rools.register(...);
-rools.evaluate(facts);
+await rools.evaluate(facts);
 ```
 
 Sometimes, it is handy to combine facts using ES6 shorthand notation:
@@ -285,26 +308,19 @@ const weather = {
 };
 const rools = new Rools();
 rools.register(...);
-rools.evaluate({ user, weather });
+await rools.evaluate({ user, weather });
 ```
 
-*Important*: Please note that rules read the facts (`when`) as well as write to the facts (`then`). Please make sure you provide a fresh set of facts whenever you call `evaluate()`.
+Please note that rules read the facts (`when`) as well as write to the facts (`then`).
+Please make sure you provide a fresh set of facts whenever you call `evaluate()`.
 
-If during `evaluate()`, firing actions (`then`) or evaluating premises (`when`) raise errors, `evaluate()` will *not* fail. However, the errors are passed to its logger, which you can hook into like this.
-
-```js
-const delegate = ({ message, rule, error }) => {
-  console.error(message, rule, error);
-};
-const rools = new Rools({ logging: { delegate } });
-...
-rools.evaluate(facts);
-```
+`evaluate()` is working asynchronously, i.e., it returns a promise.
+If a premise (`when`) fails, `evaluate()` will still *not* fail (for robustness reasons).
+If an action (`then`) fails, `evaluate()` will reject its promise.
 
 ### Todos
 
 Some of the features on my list are:
  * Conflict resolution by specificity
- * Asynchronous actions (`then`)
  * Action/rule groups
  * More unit tests
diff --git a/package.json b/package.json
index e621416..0a7455b 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,7 @@
   "devDependencies": {
     "chai": "^4.1.2",
     "chai-as-promised": "^7.1.1",
+    "chai-shallow-deep-equal": "^1.4.6",
     "coveralls": "^3.0.0",
     "eslint": "^4.14.0",
     "eslint-config-airbnb-base": "^12.1.0",
diff --git a/src/Rools.js b/src/Rools.js
index 2a6a72e..25dd39d 100644
--- a/src/Rools.js
+++ b/src/Rools.js
@@ -54,8 +54,9 @@ class Rools {
     assert(rule.then, `"rule.then" is required "${rule.name}"`);
   }
 
-  evaluate(facts) {
+  async evaluate(facts) {
     // init
+    this.logger.log({ type: 'debug', message: 'evaluate init' });
     const memory = {}; // working memory
     this.actions.forEach((action) => {
       memory[action.id] = { ready: false, fired: false };
@@ -68,18 +69,15 @@ class Rools {
     const activeSegments = new Set();
     const premisesBySegment = {}; // hash
     // match-resolve-act cycle
-    for (
-      let step = 0;
-      step < this.maxSteps &&
-        !this.evaluateStep(proxy, delegator, memory, activeSegments, premisesBySegment, step)
-          .next().done;
-      step += 1
-    ) ;
-    // for convenience only
+    for (let step = 0; step < this.maxSteps; step += 1) {
+      const goOn = // eslint-disable-next-line no-await-in-loop
+        await this.evaluateStep(proxy, delegator, memory, activeSegments, premisesBySegment, step);
+      if (!goOn) break; // for
+    }
     return facts;
   }
 
-  * evaluateStep(facts, delegator, memory, activeSegments, premisesBySegment, step) {
+  async evaluateStep(facts, delegator, memory, activeSegments, premisesBySegment, step) {
     this.logger.log({ type: 'debug', message: `evaluate step ${step}` });
     // create agenda for premises
     const premisesAgenda = step === 0 ? this.premises : new Set();
@@ -102,7 +100,7 @@ class Rools {
           premises.add(premise); // might grow for "hidden" conditions
         });
         memory[premise.id].value = premise.when(facts); // >>> evaluate premise!
-      } catch (error) {
+      } catch (error) { // ignore error!
         memory[premise.id].value = undefined;
         this.logger.log({
           type: 'error', message: 'error in premise (when)', rule: premise.name, error,
@@ -135,7 +133,7 @@ class Rools {
     const action = this.evaluateSelect(conflictSet);
     if (!action) {
       this.logger.log({ type: 'debug', message: 'evaluation complete' });
-      return; // done
+      return false; // done
     }
     // fire action
     this.logger.log({ type: 'debug', message: 'fire action', rule: action.name });
@@ -146,11 +144,12 @@ class Rools {
         this.logger.log({ type: 'debug', message: `write "${segment}"`, rule: action.name });
         activeSegments.add(segment);
       });
-      action.then(facts); // >>> fire action!
-    } catch (error) {
+      await this.fire(action, facts); // >>> fire action!
+    } catch (error) { // re-throw error!
       this.logger.log({
         type: 'error', message: 'error in action (then)', rule: action.name, error,
       });
+      throw new Error(`error in action (then): ${action.name}`, error);
     } finally {
       delegator.unset();
     }
@@ -159,10 +158,10 @@ class Rools {
       this.logger.log({
         type: 'debug', message: 'evaluation stop after final rule', rule: action.name,
       });
-      return; // done
+      return false; // done
     }
     // next step
-    yield; // not yet done
+    return true; // not yet done
   }
 
   evaluateSelect(actions) {
@@ -179,6 +178,15 @@ class Rools {
     this.logger.log({ type: 'debug', message: 'conflict resolution by priority' });
     return actionsWithPrio[0];
   }
+
+  async fire(action, facts) { // eslint-disable-line class-methods-use-this
+    try {
+      const thenable = action.then(facts); // >>> fire action!
+      return thenable && thenable.then ? thenable : Promise.resolve();
+    } catch (error) {
+      return Promise.reject(error);
+    }
+  }
 }
 
 module.exports = Rools;
diff --git a/test/async.spec.js b/test/async.spec.js
new file mode 100644
index 0000000..f7f6c32
--- /dev/null
+++ b/test/async.spec.js
@@ -0,0 +1,22 @@
+const Rools = require('../src');
+const { frank } = require('./facts/users')();
+const { rule1, rule2 } = require('./rules/availability');
+require('./setup');
+
+describe('Rules.evaluate() / async', () => {
+  it('should call async action / action with async/await', async () => {
+    const rools = new Rools({ logging: { debug: true } });
+    rools.register(rule1);
+    const result = await rools.evaluate({ user: frank });
+    console.log(result); // eslint-disable-line no-console
+    expect(result.products).to.shallowDeepEqual(['dsl', 'm4g', 'm3g']);
+  });
+
+  it('should call async action / action with promises', async () => {
+    const rools = new Rools({ logging: { debug: true } });
+    rools.register(rule2);
+    const result = await rools.evaluate({ user: frank });
+    console.log(result); // eslint-disable-line no-console
+    expect(result.products).to.shallowDeepEqual(['dsl', 'm4g', 'm3g']);
+  });
+});
diff --git a/test/errors.spec.js b/test/errors.spec.js
index 1644e72..9c02a6b 100644
--- a/test/errors.spec.js
+++ b/test/errors.spec.js
@@ -1,3 +1,4 @@
+const assert = require('assert');
 const Rools = require('../src');
 const { frank } = require('./facts/users')();
 const { good } = require('./facts/weather')();
@@ -7,7 +8,7 @@ const {
 require('./setup');
 
 describe('Rules.evaluate()', () => {
-  it('should not fail if `when` throws error', () => {
+  it('should not fail if `when` throws error', async () => {
     const brokenRule = {
       name: 'broken rule #1',
       when: facts => facts.bla.blub === 'blub', // TypeError: Cannot read property 'blub' of undefined
@@ -16,12 +17,16 @@ describe('Rules.evaluate()', () => {
     const rools = new Rools({ logging: { error: false } });
     const facts = { user: frank, weather: good };
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    expect(() => rools.evaluate(facts)).to.not.throw();
+    try {
+      await rools.evaluate(facts);
+    } catch (error) {
+      assert.fail();
+    }
     expect(facts.user.mood).to.be.equal('great');
     expect(facts.goWalking).to.be.equal(true);
   });
 
-  it('should not fail if `then` throws error', () => {
+  it('should fail if `then` throws error', async () => {
     const brokenRule = {
       name: 'broken rule #2',
       when: () => true, // fire immediately
@@ -32,8 +37,11 @@ describe('Rules.evaluate()', () => {
     const rools = new Rools({ logging: { error: false } });
     const facts = { user: frank, weather: good };
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    expect(() => rools.evaluate(facts)).to.not.throw();
-    expect(facts.user.mood).to.be.equal('great');
-    expect(facts.goWalking).to.be.equal(true);
+    try {
+      await rools.evaluate(facts);
+      assert.fail();
+    } catch (error) {
+      // ignore
+    }
   });
 });
diff --git a/test/facts/users.js b/test/facts/users.js
index 072cfee..c7237c1 100644
--- a/test/facts/users.js
+++ b/test/facts/users.js
@@ -1,14 +1,26 @@
 const frank = {
   name: 'frank',
   stars: 347,
+  dateOfBirth: new Date('1995-01-01'),
+  address: {
+    city: 'hamburg',
+    street: 'redderkoppel',
+    country: 'germany',
+  },
 };
 
 const michael = {
   name: 'michael',
   stars: 156,
+  dateOfBirth: new Date('1999-08-08'),
+  address: {
+    city: 'san Francisco',
+    street: 'willard',
+    country: 'usa',
+  },
 };
 
 module.exports = () => ({
-  frank: { ...frank },
-  michael: { ...michael },
+  frank: JSON.parse(JSON.stringify(frank)),
+  michael: JSON.parse(JSON.stringify(michael)),
 });
diff --git a/test/facts/weather.js b/test/facts/weather.js
index f1a2976..30bc0b6 100644
--- a/test/facts/weather.js
+++ b/test/facts/weather.js
@@ -1,16 +1,20 @@
 const good = {
   temperature: 20,
+  humidity: 39,
+  pressure: 1319,
   windy: true,
   rainy: false,
 };
 
 const bad = {
   temperature: 9,
+  humidity: 89,
+  pressure: 1013,
   windy: true,
   rainy: true,
 };
 
 module.exports = () => ({
-  good: { ...good },
-  bad: { ...bad },
+  good: JSON.parse(JSON.stringify(good)),
+  bad: JSON.parse(JSON.stringify(bad)),
 });
diff --git a/test/final.spec.js b/test/final.spec.js
index 78a1a43..c2da2dc 100644
--- a/test/final.spec.js
+++ b/test/final.spec.js
@@ -19,8 +19,8 @@ describe('Rules.evaluate() / final rule', () => {
     );
   });
 
-  it('should terminate after final rule', () => {
-    const result = rools.evaluate({ user: frank, weather: good });
+  it('should terminate after final rule', async () => {
+    const result = await rools.evaluate({ user: frank, weather: good });
     expect(result.user.mood).to.be.equal('great');
     expect(result.goWalking).to.be.equal(undefined);
     expect(result.stayAtHome).to.be.equal(undefined);
diff --git a/test/logging.spec.js b/test/logging.spec.js
index b6a6aa8..3753ecd 100644
--- a/test/logging.spec.js
+++ b/test/logging.spec.js
@@ -7,18 +7,18 @@ const {
 require('./setup');
 
 describe('Rules.evaluate() / delegate logging', () => {
-  it('should log debug', () => {
+  it('should log debug', async () => {
     let counter = 0;
     const spy = () => {
       counter += 1;
     };
     const rools = new Rools({ logging: { error: false, debug: true, delegate: spy } });
     rools.register(ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    await rools.evaluate({ user: frank, weather: good });
     expect(counter).to.not.be.equals(0);
   });
 
-  it('should log errors', () => {
+  it('should log errors', async () => {
     const brokenRule = {
       name: 'broken rule #2',
       when: () => true, // fire immediately
@@ -32,11 +32,15 @@ describe('Rules.evaluate() / delegate logging', () => {
     };
     const rools = new Rools({ logging: { error: true, debug: false, delegate: spy } });
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    try {
+      await rools.evaluate({ user: frank, weather: good });
+    } catch (error) {
+      // ignore
+    }
     expect(counter).to.not.be.equals(0);
   });
 
-  it('should log errors by default', () => {
+  it('should log errors by default', async () => {
     const brokenRule = {
       name: 'broken rule #2',
       when: () => true, // fire immediately
@@ -50,7 +54,11 @@ describe('Rules.evaluate() / delegate logging', () => {
     };
     const rools = new Rools({ logging: { delegate: spy } });
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    try {
+      await rools.evaluate({ user: frank, weather: good });
+    } catch (error) {
+      // ignore
+    }
     expect(counter).to.not.be.equals(0);
   });
 });
@@ -66,14 +74,14 @@ describe('Rules.evaluate() / console logging', () => {
     console.error.restore(); // eslint-disable-line no-console
   });
 
-  it('should log debug', () => {
+  it('should log debug', async () => {
     const rools = new Rools({ logging: { error: false, debug: true } });
     rools.register(ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    await rools.evaluate({ user: frank, weather: good });
     expect(console.log).to.be.called; // eslint-disable-line no-unused-expressions, no-console
   });
 
-  it('should log errors', () => {
+  it('should log errors', async () => {
     const brokenRule = {
       name: 'broken rule #2',
       when: () => true, // fire immediately
@@ -83,11 +91,15 @@ describe('Rules.evaluate() / console logging', () => {
     };
     const rools = new Rools({ logging: { error: true, debug: false } });
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    try {
+      await rools.evaluate({ user: frank, weather: good });
+    } catch (error) {
+      // ignore
+    }
     expect(console.error).to.be.called; // eslint-disable-line no-unused-expressions, no-console
   });
 
-  it('should log errors by default', () => {
+  it('should log errors by default', async () => {
     const brokenRule = {
       name: 'broken rule #2',
       when: () => true, // fire immediately
@@ -97,11 +109,15 @@ describe('Rules.evaluate() / console logging', () => {
     };
     const rools = new Rools();
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    try {
+      await rools.evaluate({ user: frank, weather: good });
+    } catch (error) {
+      // ignore
+    }
     expect(console.error).to.be.called; // eslint-disable-line no-unused-expressions, no-console
   });
 
-  it('should log errors by default / 2', () => {
+  it('should log errors by default / 2', async () => {
     const brokenRule = {
       name: 'broken rule #2',
       when: () => true, // fire immediately
@@ -111,7 +127,11 @@ describe('Rules.evaluate() / console logging', () => {
     };
     const rools = new Rools({});
     rools.register(brokenRule, ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
-    rools.evaluate({ user: frank, weather: good });
+    try {
+      await rools.evaluate({ user: frank, weather: good });
+    } catch (error) {
+      // ignore
+    }
     expect(console.error).to.be.called; // eslint-disable-line no-unused-expressions, no-console
   });
 });
diff --git a/test/longer.spec.js b/test/longer.spec.js
index 731f4a5..2a6404d 100644
--- a/test/longer.spec.js
+++ b/test/longer.spec.js
@@ -79,12 +79,12 @@ describe('Rules.evaluate() / longer cycle', () => {
     rools.register(rule7, rule0, rule2, rule3, rule1, rule6, rule8, rule4, rule9, rule5);
   });
 
-  it('should fire 10 rules', () => {
+  it('should fire 10 rules', async () => {
     const frank = {
       name: 'frank',
       stars: 0,
     };
-    const result = rools.evaluate({ user: frank });
+    const result = await rools.evaluate({ user: frank });
     expect(result.user.stars).to.be.equal(10);
   });
 });
diff --git a/test/rules/availability.js b/test/rules/availability.js
new file mode 100644
index 0000000..9b5612e
--- /dev/null
+++ b/test/rules/availability.js
@@ -0,0 +1,30 @@
+const availabilityCheck = (address) => { // eslint-disable-line arrow-body-style
+  return new Promise((resolve) => {
+    setTimeout(() => {
+      if (address.country === 'germany') {
+        if (address.city === 'hamburg') {
+          return resolve(['dsl', 'm4g', 'm3g']);
+        }
+      }
+      return resolve([]);
+    }, 100);
+  });
+};
+
+const rule1 = {
+  name: 'check availability of products (async await)',
+  when: facts => facts.user.address.country === 'germany',
+  then: async (facts) => {
+    facts.products = await availabilityCheck(facts.user.address);
+  },
+};
+
+const rule2 = {
+  name: 'check availability of products (promises)',
+  when: facts => facts.user.address.country === 'germany',
+  then: facts =>
+    availabilityCheck(facts.user.address)
+      .then((result) => { facts.products = result; }),
+};
+
+module.exports = { rule1, rule2 };
diff --git a/test/setup.js b/test/setup.js
index 7389f34..24030b6 100644
--- a/test/setup.js
+++ b/test/setup.js
@@ -1,9 +1,11 @@
 const chai = require('chai');
 const chaiAsPromised = require('chai-as-promised');
+const chaiShallowDeepEqual = require('chai-shallow-deep-equal');
 const sinon = require('sinon');
 const sinonChai = require('sinon-chai');
 
 chai.use(chaiAsPromised);
+chai.use(chaiShallowDeepEqual);
 chai.use(sinonChai);
 
 global.chai = chai;
diff --git a/test/simple.spec.js b/test/simple.spec.js
index bd9157b..353d50d 100644
--- a/test/simple.spec.js
+++ b/test/simple.spec.js
@@ -14,24 +14,24 @@ describe('Rules.evaluate() / simple scenarios', () => {
     rools.register(ruleMoodGreat, ruleMoodSad, ruleGoWalking, ruleStayAtHome);
   });
 
-  it('Test 1', () => {
-    const result = rools.evaluate({ user: frank, weather: good });
+  it('Test 1', async () => {
+    const result = await rools.evaluate({ user: frank, weather: good });
     // console.log(result); // eslint-disable-line no-console
     expect(result.user.mood).to.be.equal('great');
     expect(result.goWalking).to.be.equal(true);
     expect(result.stayAtHome).to.be.equal(undefined);
   });
 
-  it('Test 2', () => {
-    const result = rools.evaluate({ user: michael, weather: good });
+  it('Test 2', async () => {
+    const result = await rools.evaluate({ user: michael, weather: good });
     // console.log(result); // eslint-disable-line no-console
     expect(result.user.mood).to.be.equal('sad');
     expect(result.goWalking).to.be.equal(undefined);
     expect(result.stayAtHome).to.be.equal(true);
   });
 
-  it('Test 3', () => {
-    const result = rools.evaluate({ user: frank, weather: bad });
+  it('Test 3', async () => {
+    const result = await rools.evaluate({ user: frank, weather: bad });
     // console.log(result); // eslint-disable-line no-console
     expect(result.user.mood).to.be.equal('great');
     expect(result.goWalking).to.be.equal(undefined);