diff --git a/ReadMe.md b/ReadMe.md
index 2a217cb..a044daf 100644
--- a/ReadMe.md
+++ b/ReadMe.md
@@ -47,7 +47,7 @@ then use [Artemis-odb](https://github.com/junkdog/artemis-odb) or [Ashley](https
 
 ## Current Status
 
-Release Candidate version 1.0-RC2 is available on maven central since 19-Jan-2022. Please feel free to contribute to the
+Release Candidate version 1.0-RC3 is available on maven central since 30-Jan-2022. Please feel free to contribute to the
 Discussions or Issues. Help is always appreciated. 
 To use Fleks add it as a dependency to your project:
 
@@ -57,20 +57,20 @@ To use Fleks add it as a dependency to your project:
 <dependency>
   <groupId>io.github.quillraven.fleks</groupId>
   <artifactId>Fleks</artifactId>
-  <version>1.0-RC2</version>
+  <version>1.0-RC3</version>
 </dependency>
 ```
 
 #### Gradle (Groovy)
 
 ```kotlin
-implementation 'io.github.quillraven.fleks:Fleks:1.0-RC2'
+implementation 'io.github.quillraven.fleks:Fleks:1.0-RC3'
 ```
 
 #### Gradle (Kotlin)
 
 ```kotlin
-implementation("io.github.quillraven.fleks:Fleks:1.0-RC2")
+implementation("io.github.quillraven.fleks:Fleks:1.0-RC3")
 ```
 
 ## Current API and usage
@@ -233,7 +233,11 @@ class AnimationSystem(
 
 If you need to modify the component configuration of an entity then this can be done via the `configureEntity` function
 of an `IteratingSystem`. The purpose of this function is performance reasons to trigger internal calculations
-of Fleks only once instead of each time a component gets added or removed.
+of Fleks only once instead of each time a component gets added or removed. Inside `configureEntity` you get access to
+three special `ComponentMapper` functions:
+- `add`: adds a component to an entity
+- `remove`: removes a component from an entity
+- `addOrUpdate`: adds a component only if it does not exist yet. Otherwise, it just updates the existing component
 
 Let's see how a system can look like that adds a `DeadComponent` to an entity and removes a `LifeComponent` when its
 hitpoints are <= 0:
@@ -367,6 +371,25 @@ fun main() {
 }
 ```
 
+If you ever need to iterate over entities outside a system then this is also possible but please note that
+systems are always the preferred way of iteration in an entity component system.
+The world's `forEach` function allows you to iterate over all active entities:
+
+```Kotlin
+ fun main() {
+    val world = World {}
+    val e1 = world.entity()
+    val e2 = world.entity()
+    val e3 = world.entity()
+    world.remove(e2)
+
+    // this will iterate over entities e1 and e3
+    world.forEach { entity ->
+        // do something with the entity
+    }
+}
+```
+
 ### Entity and Components
 
 We now know how to create a world and add systems to it, but we don't know how to add entities to our world. This can be
diff --git a/build.gradle.kts b/build.gradle.kts
index a23c8a9..bca8f0b 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -7,7 +7,7 @@ plugins {
 }
 
 group = "io.github.quillraven.fleks"
-version = "1.0-RC2"
+version = "1.0-RC3"
 java.sourceCompatibility = JavaVersion.VERSION_1_8
 
 val bmSourceSetName = "benchmarks"
diff --git a/src/main/kotlin/com/github/quillraven/fleks/component.kt b/src/main/kotlin/com/github/quillraven/fleks/component.kt
index 0cab4ea..3abfa48 100644
--- a/src/main/kotlin/com/github/quillraven/fleks/component.kt
+++ b/src/main/kotlin/com/github/quillraven/fleks/component.kt
@@ -60,6 +60,20 @@ class ComponentMapper<T>(
         }
     }
 
+    /**
+     * Creates a new component if the [entity] does not have it yet. Otherwise, updates the existing component.
+     * Applies the [configuration] in both cases and returns the component.
+     * Notifies any registered [ComponentListener] if a new component is created.
+     */
+    @PublishedApi
+    internal inline fun addOrUpdateInternal(entity: Entity, configuration: T.() -> Unit = {}): T {
+        return if (entity in this) {
+            this[entity].apply(configuration)
+        } else {
+            addInternal(entity, configuration)
+        }
+    }
+
     /**
      * Removes a component of the specific type from the given [entity].
      * Notifies any registered [ComponentListener].
diff --git a/src/main/kotlin/com/github/quillraven/fleks/entity.kt b/src/main/kotlin/com/github/quillraven/fleks/entity.kt
index b33cb88..c0da713 100644
--- a/src/main/kotlin/com/github/quillraven/fleks/entity.kt
+++ b/src/main/kotlin/com/github/quillraven/fleks/entity.kt
@@ -47,6 +47,7 @@ class EntityCreateCfg(
     /**
      * Adds and returns a component of the given type to the [entity] and
      * applies the [configuration] to the component.
+     * Notifies any registered [ComponentListener].
      *
      * @throws [FleksMissingNoArgsComponentConstructorException] if the component of the given type
      * does not have a no argument constructor.
@@ -72,14 +73,27 @@ class EntityUpdateCfg {
      * Adds and returns a component of the given type to the [entity] and applies the [configuration] to that component.
      * If the [entity] already has a component of the given type then no new component is created and instead
      * the existing one will be updated.
+     * Notifies any registered [ComponentListener].
      */
     inline fun <reified T : Any> ComponentMapper<T>.add(entity: Entity, configuration: T.() -> Unit = {}): T {
         cmpMask.set(this.id)
         return this.addInternal(entity, configuration)
     }
 
+    /**
+     * Adds a new component of the given type to the [entity] if it does not have it yet.
+     * Otherwise, updates the already existing component.
+     * Applies the [configuration] in both cases and returns the component.
+     * Notifies any registered [ComponentListener] if a new component is created.
+     */
+    inline fun <reified T : Any> ComponentMapper<T>.addOrUpdate(entity: Entity, configuration: T.() -> Unit = {}): T {
+        cmpMask.set(this.id)
+        return this.addOrUpdateInternal(entity, configuration)
+    }
+
     /**
      * Removes a component of the given type from the [entity].
+     * Notifies any registered [ComponentListener].
      *
      * @throws [ArrayIndexOutOfBoundsException] if the id of the [entity] exceeds the mapper's capacity.
      */
@@ -240,6 +254,19 @@ class EntityService(
         }
     }
 
+    /**
+     * Performs the given [action] on each active [entity][Entity].
+     */
+    fun forEach(action: (Entity) -> Unit) {
+        for (id in 0 until nextId) {
+            val entity = Entity(id)
+            if (removedEntities[entity.id]) {
+                continue
+            }
+            entity.run(action)
+        }
+    }
+
     /**
      * Clears the [delayRemoval] flag and removes [entities][Entity] which are part of the [delayedEntities].
      */
diff --git a/src/main/kotlin/com/github/quillraven/fleks/world.kt b/src/main/kotlin/com/github/quillraven/fleks/world.kt
index 6f019a8..6e13e44 100644
--- a/src/main/kotlin/com/github/quillraven/fleks/world.kt
+++ b/src/main/kotlin/com/github/quillraven/fleks/world.kt
@@ -177,6 +177,13 @@ class World(
         entityService.removeAll()
     }
 
+    /**
+     * Performs the given [action] on each active [entity][Entity].
+     */
+    fun forEach(action: (Entity) -> Unit) {
+        entityService.forEach(action)
+    }
+
     /**
      * Returns the specified [system][IntervalSystem] of the world.
      *
diff --git a/src/test/kotlin/com/github/quillraven/fleks/ComponentTest.kt b/src/test/kotlin/com/github/quillraven/fleks/ComponentTest.kt
index 283f865..8c1af2c 100644
--- a/src/test/kotlin/com/github/quillraven/fleks/ComponentTest.kt
+++ b/src/test/kotlin/com/github/quillraven/fleks/ComponentTest.kt
@@ -229,4 +229,34 @@ internal class ComponentTest {
             { assertEquals("add", listener.lastCall) }
         )
     }
+
+    @Test
+    fun `add component if it does not exist yet`() {
+        val cmpService = ComponentService()
+        val mapper = cmpService.mapper<ComponentTestComponent>()
+        val entity = Entity(1)
+
+        val cmp = mapper.addOrUpdateInternal(entity) { x++ }
+
+        assertAll(
+            { assertTrue(entity in mapper) },
+            { assertEquals(1, cmp.x) }
+        )
+    }
+
+    @Test
+    fun `update component if it already exists`() {
+        val cmpService = ComponentService()
+        val mapper = cmpService.mapper<ComponentTestComponent>()
+        val entity = Entity(1)
+        val expectedCmp = mapper.addOrUpdateInternal(entity) { x++ }
+
+        val actualCmp = mapper.addOrUpdateInternal(entity) { x++ }
+
+        assertAll(
+            { assertTrue(entity in mapper) },
+            { assertEquals(expectedCmp, actualCmp) },
+            { assertEquals(2, actualCmp.x) }
+        )
+    }
 }
diff --git a/src/test/kotlin/com/github/quillraven/fleks/EntityTest.kt b/src/test/kotlin/com/github/quillraven/fleks/EntityTest.kt
index 4a6993a..4ad687e 100644
--- a/src/test/kotlin/com/github/quillraven/fleks/EntityTest.kt
+++ b/src/test/kotlin/com/github/quillraven/fleks/EntityTest.kt
@@ -119,6 +119,27 @@ internal class EntityTest {
         )
     }
 
+    @Test
+    fun `update component of entity if it already exists with custom listener`() {
+        val cmpService = ComponentService()
+        val entityService = EntityService(32, cmpService)
+        val listener = EntityTestListener()
+        val expectedEntity = entityService.create { }
+        val mapper = cmpService.mapper<EntityTestComponent>()
+        entityService.addEntityListener(listener)
+
+        entityService.configureEntity(expectedEntity) {
+            mapper.add(expectedEntity) { ++x }
+            mapper.addOrUpdate(expectedEntity) { x++ }
+        }
+
+        assertAll(
+            { assertTrue(expectedEntity in mapper) },
+            { assertEquals(2f, mapper[expectedEntity].x) },
+            { assertEquals(1, listener.numCalls) }
+        )
+    }
+
     @Test
     fun `remove entity with a component immediately with custom listener`() {
         val cmpService = ComponentService()
diff --git a/src/test/kotlin/com/github/quillraven/fleks/WorldTest.kt b/src/test/kotlin/com/github/quillraven/fleks/WorldTest.kt
index 4e245e3..9d4ff28 100644
--- a/src/test/kotlin/com/github/quillraven/fleks/WorldTest.kt
+++ b/src/test/kotlin/com/github/quillraven/fleks/WorldTest.kt
@@ -4,6 +4,7 @@ import org.junit.jupiter.api.Assertions.assertAll
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.assertThrows
+import kotlin.test.assertContentEquals
 import kotlin.test.assertNotNull
 import kotlin.test.assertTrue
 
@@ -255,4 +256,18 @@ internal class WorldTest {
             }
         }
     }
+
+    @Test
+    fun `iterate over all active entities`() {
+        val w = World {}
+        val e1 = w.entity()
+        val e2 = w.entity()
+        val e3 = w.entity()
+        w.remove(e2)
+        val actualEntities = mutableListOf<Entity>()
+
+        w.forEach { actualEntities.add(it) }
+
+        assertContentEquals(listOf(e1, e3), actualEntities)
+    }
 }