diff --git a/packages/ecs/src/ecs.js b/packages/ecs/src/ecs.js
index 5bd0217..537ad3f 100644
--- a/packages/ecs/src/ecs.js
+++ b/packages/ecs/src/ecs.js
@@ -21,7 +21,21 @@ export default class Ecs {
   }
 
   addSystem(System) {
-    const system = new System(this.Components);
+    const ecs = this;
+    class WrappedSystem extends System {
+
+      // eslint-disable-next-line class-methods-use-this
+      createEntity(components) {
+        return ecs.create(components);
+      }
+
+      // eslint-disable-next-line class-methods-use-this
+      createManyEntities(count, components) {
+        return ecs.createMany(count, components);
+      }
+
+    }
+    const system = new WrappedSystem(this.Components);
     this.$$systems.push(system);
     return system;
   }
@@ -164,13 +178,14 @@ export default class Ecs {
   }
 
   encode(entities, view) {
+    let cursor = 0;
+    let entitiesWritten = 0;
+    view.setUint32(cursor, entities.length, true);
+    cursor += 4;
     if (0 === entities.length) {
       return;
     }
     const keys = Object.keys(this.Components);
-    let cursor = 0;
-    view.setUint32(cursor, entities.length, true);
-    cursor += 4;
     for (let i = 0; i < entities.length; i++) {
       let entity;
       let onlyDirty = false;
@@ -183,6 +198,7 @@ export default class Ecs {
       if (onlyDirty && !this.dirty.has(entity)) {
         continue;
       }
+      entitiesWritten += 1;
       view.setBigUint64(cursor, BigInt(entity), true);
       cursor += 8;
       const components = this.$$entities[entity];
@@ -208,9 +224,13 @@ export default class Ecs {
       }
       view.setUint16(componentsWrittenIndex, componentsWritten, true);
     }
+    view.setUint32(0, entitiesWritten, true);
   }
 
   get(entity, Components = Object.keys(this.Components)) {
+    if (!this.$$entities[entity]) {
+      return undefined;
+    }
     const result = {};
     for (let i = 0; i < Components.length; i++) {
       const component = this.Components[Components[i]].get(entity);
diff --git a/packages/ecs/src/index.js b/packages/ecs/src/index.js
index 33af058..1ebb1ec 100644
--- a/packages/ecs/src/index.js
+++ b/packages/ecs/src/index.js
@@ -1,113 +1,3 @@
-/* eslint-disable */
-
-import Component from './component';
-import Ecs from './ecs';
-import Schema from './schema';
-import Serializer from './serializer';
-import System from './system';
-
-const N = 1000;
-const warm = 500;
-
-const marks = [];
-
-function mark(label, fn) {
-  marks.push(label);
-  performance.mark(`${label}-before`);
-  fn();
-  performance.mark(`${label}-after`);
-}
-
-function measure() {
-  for (let i = 0; i < marks.length; i++) {
-    const label = marks[i];
-    performance.measure(label, `${label}-before`, `${label}-after`);
-  }
-  console.log(performance.getEntriesByType('measure').map(({duration, name}) => ({duration, name})));
-}
-
-class Position extends Component {
-
-  static schema = {
-    x: {type: 'int32', defaultValue: 32},
-    y: 'int32',
-    z: 'int32',
-  };
-
-}
-
-class Direction extends Component {
-
-  static schema = {
-    direction: 'uint8',
-  };
-
-}
-
-class ZSystem extends System {
-
-  static queries() {
-    return {
-      default: ['Position', 'Direction'],
-    };
-  }
-
-  tick() {
-    for (let [position, {direction}, entity] of this.select('default')) {
-      position.z = entity * direction * 2;
-    }
-  }
-
-}
-
-const ecs = new Ecs({Direction, Position});
-ecs.addSystem(ZSystem);
-
-const createMany = () => {
-  const entities = ecs.createMany(N, {Position: (entity) => ({y: entity})});
-  ecs.insertMany({Direction: entities.map((entity) => [entity, {direction: 1 + entity % 4}])});
-  return entities;
-};
-
-for (let i = 0; i < warm; ++i) {
-  ecs.destroyMany(createMany());
-}
-mark('create', createMany);
-
-let buffer, view;
-
-ecs.tick(0.01);
-ecs.tick(0.01);
-if (!view) {
-  buffer = new ArrayBuffer(ecs.sizeOf(Array.from(ecs.dirty.values()), false));
-  view = new DataView(buffer);
-}
-console.log('bytes:', buffer.byteLength);
-const encoding = Array.from(ecs.dirty.values());
-ecs.tickFinalize();
-
-console.log('encoding', encoding.length);
-
-for (let i = 0; i < warm; ++i) {
-  ecs.tick(0.01);
-  ecs.encode(encoding, view);
-  ecs.tickFinalize();
-}
-
-mark('tick', () => ecs.tick(0.01));
-mark('encode', () => ecs.encode(encoding, view));
-mark('finalize', () => ecs.tickFinalize());
-
-ecs.destroyAll();
-
-for (let i = 0; i < warm; ++i) {
-  ecs.decode(view);
-  ecs.destroyAll();
-}
-
-mark('decode', () => ecs.decode(view));
-
-console.log(JSON.stringify(ecs.get(1)));
-
-console.log(N, 'iterations');
-measure();
+export {default as Component} from './component';
+export {default as Ecs} from './ecs';
+export {default as System} from './system';
diff --git a/packages/ecs/src/system.js b/packages/ecs/src/system.js
index 580caa4..4c97ffb 100644
--- a/packages/ecs/src/system.js
+++ b/packages/ecs/src/system.js
@@ -31,6 +31,10 @@ export default class System {
     }
   }
 
+  static queries() {
+    return {};
+  }
+
   reindex(entities) {
     for (const i in this.queries) {
       this.queries[i].reindex(entities);
diff --git a/packages/ecs/test/ecs.js b/packages/ecs/test/ecs.js
new file mode 100644
index 0000000..8d8fee7
--- /dev/null
+++ b/packages/ecs/test/ecs.js
@@ -0,0 +1,126 @@
+import {expect} from 'chai';
+
+import Component from '../src/component';
+import Ecs from '../src/ecs';
+import System from '../src/system';
+
+// eslint-disable-next-line
+class Empty extends Component {}
+
+class Position extends Component {
+
+  static schema = {
+    x: {type: 'int32', defaultValue: 32},
+    y: 'int32',
+    z: 'int32',
+  };
+
+}
+
+it('can create entities with components', () => {
+  const ecs = new Ecs({Empty, Position});
+  const entity = ecs.create({Empty: () => {}, Position: () => ({y: 420})});
+  expect(JSON.stringify(ecs.get(entity)))
+    .to.deep.equal(JSON.stringify({Empty: {}, Position: {x: 32, y: 420, z: 0}}));
+});
+
+it('can tick systems', () => {
+  class Momentum extends Component {
+
+    static schema = {
+      x: 'int32',
+      y: 'int32',
+      z: 'int32',
+    };
+
+  }
+  const ecs = new Ecs({Momentum, Position});
+  class Physics extends System {
+
+    static queries() {
+      return {
+        default: ['Position', 'Momentum'],
+      };
+    }
+
+    tick(elapsed) {
+      expect(elapsed)
+        .to.equal(1);
+      // eslint-disable-next-line no-restricted-syntax
+      for (const [position, momentum] of this.select('default')) {
+        position.x += momentum.x * elapsed;
+        position.y += momentum.y * elapsed;
+        position.z += momentum.z * elapsed;
+      }
+    }
+
+  }
+  ecs.addSystem(Physics);
+  const entity = ecs.create({Momentum: () => ({}), Position: () => ({y: 420})});
+  const position = JSON.stringify(ecs.get(entity).Position);
+  ecs.tick(1);
+  expect(JSON.stringify(ecs.get(entity).Position))
+    .to.deep.equal(position);
+  ecs.get(1).Momentum.y = 30;
+  ecs.tick(1);
+  expect(JSON.stringify(ecs.get(entity).Position))
+    .to.deep.equal(JSON.stringify({x: 32, y: 450, z: 0}));
+});
+
+it('can create entities when ticking systems', () => {
+  const ecs = new Ecs();
+  class Spawn extends System {
+
+    tick() {
+      this.createEntity();
+    }
+
+  }
+  ecs.addSystem(Spawn);
+  ecs.create();
+  expect(ecs.get(2))
+    .to.be.undefined;
+  ecs.tick(1);
+  expect(ecs.get(2))
+    .to.not.be.undefined;
+});
+
+it('can schedule entities to be deleted when ticking systems', () => {
+  const ecs = new Ecs();
+  class Despawn extends System {
+
+    tick() {
+      this.destroyEntity(1);
+    }
+
+  }
+  ecs.addSystem(Despawn);
+  ecs.createExact(1);
+  ecs.tick(1);
+  expect(ecs.get(1))
+    .to.not.be.undefined;
+  ecs.tickFinalize();
+  expect(ecs.get(1))
+    .to.be.undefined;
+});
+
+it('can encode and decode an ecs', () => {
+  const ecs = new Ecs({Empty, Position});
+  const entity = ecs.create({Empty: () => {}, Position: () => ({y: 420})});
+  const view = new DataView(new ArrayBuffer(1024));
+  ecs.encode([[entity, true]], view);
+  const newEcs = new Ecs({Empty, Position});
+  newEcs.decode(view);
+  expect(JSON.stringify(newEcs.get(entity)))
+    .to.deep.equal(JSON.stringify(ecs.get(entity)));
+  ecs.setClean();
+  ecs.encode([[entity, true]], view);
+  const newEcs2 = new Ecs({Empty, Position});
+  newEcs2.decode(view);
+  expect(newEcs2.get(entity))
+    .to.be.undefined;
+  ecs.encode([entity], view);
+  newEcs2.decode(view);
+  expect(newEcs2.get(entity))
+    .to.not.be.undefined;
+});
diff --git a/packages/ecs/test/query.js b/packages/ecs/test/query.js
new file mode 100644
index 0000000..eac90d2
--- /dev/null
+++ b/packages/ecs/test/query.js
@@ -0,0 +1,61 @@
+import {expect} from 'chai';
+
+import Component from '../src/component';
+import Query from '../src/query';
+
+class A extends Component {
+
+  static schema = {
+    a: 'int32',
+  };
+
+}
+
+class B extends Component {
+
+  static schema = {
+    b: 'int32',
+  };
+
+}
+
+const Components = {A: new A(), B: new B()};
+Components.A.createMany([2, 3]);
+Components.B.createMany([1, 2]);
+function testQuery(parameters, expected) {
+  const query = new Query(parameters, Components);
+  query.reindex([1, 2, 3]);
+  expect(query.count)
+    .to.equal(expected.length);
+  // eslint-disable-next-line no-restricted-syntax
+  for (const _ of query.select()) {
+    expect(_.length)
+      .to.equal(parameters.filter((spec) => '!'.charCodeAt(0) !== spec.charCodeAt(0)).length + 1);
+    expect(expected.includes(_.pop()))
+      .to.equal(true);
+  }
+}
+
+it('can query all', () => {
+  testQuery([], [1, 2, 3]);
+});
+
+it('can query some', () => {
+  testQuery(['A'], [2, 3]);
+  testQuery(['A', 'B'], [2]);
+});
+
+it('can query excluding', () => {
+  testQuery(['!A'], [1]);
+  testQuery(['A', '!B'], [3]);
+});
+
+it('can deindex', () => {
+  const query = new Query(['A'], Components);
+  query.reindex([1, 2, 3]);
+  expect(query.count)
+    .to.equal(2);
+  query.deindex([2]);
+  expect(query.count)
+    .to.equal(1);
+});