diff --git a/lib/ecs/entity_registry.rb b/lib/ecs/entity_registry.rb
index faa4b62..14c9235 100644
--- a/lib/ecs/entity_registry.rb
+++ b/lib/ecs/entity_registry.rb
@@ -5,11 +5,12 @@ module ECS
   class EntityRegistry
     include Logging
 
-    attr_reader :entities, :entities_by_tag, :entity_components,
+    attr_reader :world, :entities, :entities_by_tag, :entity_components,
       :component_entities
 
     # :reek:DuplicateMethodCall
-    def initialize
+    def initialize(world:)
+      @world = world
       @entities = []
       @entities_by_tag = Hash.new { |hash, key| hash[key] = [] }
       @entity_components = Hash.new { |hash, key| hash[key] = [] }
diff --git a/lib/ecs/system.rb b/lib/ecs/system.rb
index 3950a6c..b3fdcc9 100644
--- a/lib/ecs/system.rb
+++ b/lib/ecs/system.rb
@@ -3,8 +3,9 @@
 module ECS
   # Basic system implementation
   #
-  # System must implement method +run+ that accepts +time_delta+ keyword
-  # argument
+  # System must implement method +process_entoty+ that accepts +entity+ and
+  # components specified with +watch_components+ method (if any) or redefine
+  # +run+ for special cases.
   #
   # @see ECS::Systems::Movement#process_tick Movement system implementation
   # @example
@@ -124,14 +125,17 @@ def debugging?
 
     private
 
+    # @api private
     def ensure_world
       raise 'World must be set. System cannot work without it.' unless world
     end
 
+    # @api private
     def log_run
       logger.debug { "Running #{self.class.name} with delta #{time_delta} ms" }
     end
 
+    # @api private
     def log_data(entity, components)
       logger.debug do
         "Processing entity #{entity} with components #{components.inspect}"
diff --git a/lib/ecs/world.rb b/lib/ecs/world.rb
index 39d7990..f57369f 100644
--- a/lib/ecs/world.rb
+++ b/lib/ecs/world.rb
@@ -12,10 +12,10 @@ class World
     # :reek:Attribute { enabled: false }
     attr_accessor :time_delta
 
-    def initialize(width:, height:, entity_registry: EntityRegistry.new)
+    def initialize(width:, height:)
       @width = width
       @height = height
-      @entity_registry = entity_registry
+      @entity_registry = EntityRegistry.new(world: self)
       @update_systems = Set.new
       @draw_systems = Set.new
 
diff --git a/spec/unit/ecs/entity_registry_spec.rb b/spec/unit/ecs/entity_registry_spec.rb
index 29c0778..1d72584 100644
--- a/spec/unit/ecs/entity_registry_spec.rb
+++ b/spec/unit/ecs/entity_registry_spec.rb
@@ -3,7 +3,8 @@
 require 'ecs_helper'
 
 RSpec.describe ECS::EntityRegistry do
-  let(:registry) { described_class.new }
+  let(:registry) { world.entity_registry }
+  let(:world) { ECS::World.new(width: 100, height: 100) }
 
   describe '#create_entity' do
     subject(:create_entity) { registry.create_entity }
diff --git a/spec/unit/ecs/system_spec.rb b/spec/unit/ecs/system_spec.rb
index bec10af..d344e91 100644
--- a/spec/unit/ecs/system_spec.rb
+++ b/spec/unit/ecs/system_spec.rb
@@ -20,13 +20,13 @@ class Bar < ECS::Component
       ->(bl) { system.entities_with(:foo, :bar, &bl) }
     end
 
-    let(:entity_registry) { ECS::EntityRegistry.new }
+    let(:world) { ECS::World.new(width: 100, height: 100) }
+    let(:entity_registry) { world.entity_registry }
     let(:entity) { entity_registry.create_entity }
     let(:foo_component) { Foo.new }
     let(:bar_component) { Bar.new }
 
     before do
-      world = instance_double('ECS::World', entity_registry: entity_registry)
       system.world = world
 
       entity_registry.add_component(entity, foo_component)
@@ -41,13 +41,13 @@ class Bar < ECS::Component
   describe '#component' do
     subject(:result) { system.component(entity, component_name) }
 
-    let(:entity_registry) { ECS::EntityRegistry.new }
+    let(:world) { ECS::World.new(width: 100, height: 100) }
+    let(:entity_registry) { world.entity_registry }
     let(:entity) { entity_registry.create_entity }
     let(:component) { Foo.new }
     let(:component_name) { :foo }
 
     before do
-      world = instance_double('ECS::World', entity_registry: entity_registry)
       system.world = world
 
       entity_registry.add_component(entity, component)