diff --git a/packages/ecs/src/ecs.js b/packages/ecs/src/ecs.js
index 6c00ebf..fbe05d1 100644
--- a/packages/ecs/src/ecs.js
+++ b/packages/ecs/src/ecs.js
@@ -1,6 +1,7 @@
 /* eslint-disable guard-for-in, max-classes-per-file, no-continue, no-restricted-syntax */
 import Bundle from './bundle';
 import Component from './component';
+import System from './system';
 
 export default class Ecs {
 
@@ -36,9 +37,9 @@ export default class Ecs {
     }
   }
 
-  addSystem(System) {
+  addSystem(source) {
     const ecs = this;
-    class WrappedSystem extends System {
+    class WrappedSystem extends System.normalize(source) {
 
       constructor(Components) {
         super(Components);
@@ -53,10 +54,15 @@ export default class Ecs {
         return this.ecs.createMany(count, components);
       }
 
+      // eslint-disable-next-line class-methods-use-this
+      get source() {
+        return source;
+      }
+
     }
-    const system = new WrappedSystem(this.Components);
-    this.$$systems.push(system);
-    return system;
+    const wrappedSystem = new WrappedSystem(this.Components);
+    wrappedSystem.reindex(Object.keys(this.$$entities));
+    this.$$systems.push(wrappedSystem);
   }
 
   create(components = {}) {
@@ -321,6 +327,13 @@ export default class Ecs {
     }
   }
 
+  removeSystem(SystemLike) {
+    const index = this.$$systems.findIndex((system) => SystemLike === system.source);
+    if (-1 !== index) {
+      this.$$systems.splice(index, 1);
+    }
+  }
+
   setClean() {
     for (const i in this.Components) {
       this.Components[i].setClean();
diff --git a/packages/ecs/src/system.js b/packages/ecs/src/system.js
index 4c97ffb..126bbea 100644
--- a/packages/ecs/src/system.js
+++ b/packages/ecs/src/system.js
@@ -1,4 +1,4 @@
-/* eslint-disable guard-for-in, no-restricted-syntax */
+/* eslint-disable guard-for-in, max-classes-per-file, no-restricted-syntax */
 
 import Query from './query';
 
@@ -31,6 +31,18 @@ export default class System {
     }
   }
 
+  static normalize(SystemLike) {
+    if (SystemLike.prototype instanceof System) {
+      return SystemLike;
+    }
+    if ('function' === typeof SystemLike) {
+      class TickingSystem extends System {}
+      TickingSystem.prototype.tick = SystemLike;
+      return TickingSystem;
+    }
+    return undefined;
+  }
+
   static queries() {
     return {};
   }
diff --git a/packages/ecs/test/ecs.js b/packages/ecs/test/ecs.js
index 7794d4e..5e37259 100644
--- a/packages/ecs/test/ecs.js
+++ b/packages/ecs/test/ecs.js
@@ -13,6 +13,34 @@ const Position = {
   z: 'int32',
 };
 
+it('can add and remove systems at runtime', () => {
+  const ecs = new Ecs();
+  let oneCount = 0;
+  let twoCount = 0;
+  const oneSystem = () => {
+    oneCount++;
+  };
+  ecs.addSystem(oneSystem);
+  ecs.tick();
+  expect(oneCount)
+    .to.equal(1);
+  const twoSystem = () => {
+    twoCount++;
+  };
+  ecs.addSystem(twoSystem);
+  ecs.tick();
+  expect(oneCount)
+    .to.equal(2);
+  expect(twoCount)
+    .to.equal(1);
+  ecs.removeSystem(oneSystem);
+  ecs.tick();
+  expect(oneCount)
+    .to.equal(2);
+  expect(twoCount)
+    .to.equal(2);
+});
+
 it('can create entities with components', () => {
   const ecs = new Ecs({Empty, Position});
   const entity = ecs.create({Empty: {}, Position: {y: 420}});