diff --git a/src/engine/EngineRuntime.cpp b/src/engine/EngineRuntime.cpp
index 1dfc923..2cc7e0f 100644
--- a/src/engine/EngineRuntime.cpp
+++ b/src/engine/EngineRuntime.cpp
@@ -23,14 +23,26 @@ EngineConfig normalizeConfig(EngineConfig config) {
 
 EngineRuntime::EngineRuntime(EngineConfig config)
     : config_(normalizeConfig(config)),
+      world_(core_, buffer_),
       frameClock_(config_) {
 }
 
+void EngineRuntime::addSystem(std::unique_ptr<EngineSystem> system) {
+    systems_.push_back(std::move(system));
+}
+
 void EngineRuntime::initialize() {
     frameClock_.reset();
+    core_ = EcsCore{};
+    buffer_ = CommandBuffer{};
     stats_ = {};
     stats_.state = EngineState::Ready;
     ++runIndex_;
+
+    for (auto& system : systems_) {
+        system->configure(world_);
+        buffer_.flush(core_);
+    }
 }
 
 void EngineRuntime::play() {
@@ -49,6 +61,8 @@ void EngineRuntime::pause() {
 
 void EngineRuntime::stop() {
     frameClock_.reset();
+    core_ = EcsCore{};
+    buffer_ = CommandBuffer{};
     stats_ = {};
     stats_.state = EngineState::Stopped;
 }
@@ -58,6 +72,11 @@ void EngineRuntime::stepFrame(double deltaSeconds) {
         initialize();
     }
 
+    if (stats_.state == EngineState::Paused) {
+        stats_.fixedStepsThisFrame = 0;
+        return;
+    }
+
     frameClock_.beginFrame(deltaSeconds);
 
     ++stats_.frameIndex;
@@ -67,6 +86,20 @@ void EngineRuntime::stepFrame(double deltaSeconds) {
         frameClock_.consumeFixedStep();
         ++stats_.fixedStepIndex;
         ++stats_.fixedStepsThisFrame;
+
+        const EngineStepContext ctx{
+            .frameIndex      = stats_.frameIndex,
+            .fixedStepIndex  = stats_.fixedStepIndex,
+            .alpha           = frameClock_.alpha(),
+            .runIndex        = runIndex_,
+            .derivedSeed     = 0,
+        };
+
+        for (auto& system : systems_) {
+            system->update(world_, ctx);
+        }
+
+        buffer_.flush(core_);
     }
 
     stats_.alpha = frameClock_.alpha();
diff --git a/src/engine/EngineRuntime.h b/src/engine/EngineRuntime.h
index 0da63c9..05c5336 100644
--- a/src/engine/EngineRuntime.h
+++ b/src/engine/EngineRuntime.h
@@ -1,7 +1,11 @@
 #pragma once
 
 #include <cstdint>
+#include <memory>
+#include <vector>
 
+#include "engine/CommandBuffer.h"
+#include "engine/EcsCore.h"
 #include "engine/EngineConfig.h"
 #include "engine/EngineStats.h"
 #include "engine/EngineSystem.h"
@@ -13,6 +17,8 @@ class EngineRuntime {
 public:
     explicit EngineRuntime(EngineConfig config = {});
 
+    void addSystem(std::unique_ptr<EngineSystem> system);
+
     void initialize();
     void play();
     void pause();
@@ -29,9 +35,12 @@ class EngineRuntime {
 private:
     EngineConfig config_;
     EngineStats stats_;
+    EcsCore core_;
+    CommandBuffer buffer_;
     EngineWorld world_;
     FrameClock frameClock_;
     std::uint64_t runIndex_{0};
+    std::vector<std::unique_ptr<EngineSystem>> systems_;
 };
 
 }  // namespace safecrowd::engine
diff --git a/src/engine/EngineSystem.h b/src/engine/EngineSystem.h
index 943b151..c8d09d6 100644
--- a/src/engine/EngineSystem.h
+++ b/src/engine/EngineSystem.h
@@ -1,10 +1,24 @@
 #pragma once
 
+#include "engine/CommandBuffer.h"
 #include "engine/EngineStepContext.h"
+#include "engine/WorldQuery.h"
 
 namespace safecrowd::engine {
 
 class EngineWorld {
+public:
+    EngineWorld() = delete;
+    explicit EngineWorld(EcsCore& core, CommandBuffer& buffer)
+        : query_(core), commands_(buffer) {}
+
+    [[nodiscard]] WorldQuery& query() noexcept { return query_; }
+    [[nodiscard]] const WorldQuery& query() const noexcept { return query_; }
+    [[nodiscard]] WorldCommands& commands() noexcept { return commands_; }
+
+private:
+    WorldQuery query_;
+    WorldCommands commands_;
 };
 
 class EngineSystem {
diff --git a/tests/EngineRuntimeTests.cpp b/tests/EngineRuntimeTests.cpp
index b40db19..f3ac57f 100644
--- a/tests/EngineRuntimeTests.cpp
+++ b/tests/EngineRuntimeTests.cpp
@@ -1,7 +1,56 @@
 #include "TestSupport.h"
 
+#include <cstddef>
+#include <memory>
+
 #include "engine/EngineRuntime.h"
 
+namespace {
+
+struct Marker {};
+
+class UpdateCounterSystem : public safecrowd::engine::EngineSystem {
+public:
+    int& count;
+    explicit UpdateCounterSystem(int& c) : count(c) {}
+    void update(safecrowd::engine::EngineWorld&, const safecrowd::engine::EngineStepContext&) override {
+        ++count;
+    }
+};
+
+class SpawnMarkerSystem : public safecrowd::engine::EngineSystem {
+public:
+    void update(safecrowd::engine::EngineWorld& world, const safecrowd::engine::EngineStepContext&) override {
+        world.commands().spawnEntity(Marker{});
+    }
+};
+
+class ConfigureSpawnMarkerSystem : public safecrowd::engine::EngineSystem {
+public:
+    void configure(safecrowd::engine::EngineWorld& world) override {
+        world.commands().spawnEntity(Marker{});
+    }
+
+    void update(safecrowd::engine::EngineWorld&, const safecrowd::engine::EngineStepContext&) override {
+    }
+};
+
+class ConfigureObserveMarkerSystem : public safecrowd::engine::EngineSystem {
+public:
+    std::size_t& count;
+
+    explicit ConfigureObserveMarkerSystem(std::size_t& c) : count(c) {}
+
+    void configure(safecrowd::engine::EngineWorld& world) override {
+        count = world.query().view<Marker>().size();
+    }
+
+    void update(safecrowd::engine::EngineWorld&, const safecrowd::engine::EngineStepContext&) override {
+    }
+};
+
+}  // namespace
+
 SC_TEST(EngineRuntimePlayAndStepUpdatesStats) {
     safecrowd::engine::EngineRuntime runtime({
         .fixedDeltaTime = 0.25,
@@ -20,6 +69,103 @@ SC_TEST(EngineRuntimePlayAndStepUpdatesStats) {
     SC_EXPECT_NEAR(stats.alpha, 0.0, 1e-9);
 }
 
+SC_TEST(EngineRuntime_RegisteredSystem_UpdateCalledOnFixedStep) {
+    int count = 0;
+    safecrowd::engine::EngineRuntime runtime({
+        .fixedDeltaTime = 0.25,
+        .maxCatchUpSteps = 4,
+        .baseSeed = 1,
+    });
+
+    runtime.addSystem(std::make_unique<UpdateCounterSystem>(count));
+    runtime.play();
+    runtime.stepFrame(0.50);  // 2 fixed steps
+
+    SC_EXPECT_EQ(count, 2);
+}
+
+SC_TEST(EngineRuntime_WorldCommands_FlushedAfterEachFixedStep) {
+    safecrowd::engine::EngineRuntime runtime({
+        .fixedDeltaTime = 0.25,
+        .maxCatchUpSteps = 4,
+        .baseSeed = 1,
+    });
+
+    runtime.addSystem(std::make_unique<SpawnMarkerSystem>());
+    runtime.play();
+    runtime.stepFrame(0.25);  // 1 fixed step
+
+    const auto entities = runtime.world().query().view<Marker>();
+    SC_EXPECT_EQ(entities.size(), std::size_t{1});
+}
+
+SC_TEST(EngineRuntime_ConfigureCommands_AreVisibleToLaterSystems) {
+    std::size_t configuredMarkerCount = 0;
+
+    safecrowd::engine::EngineRuntime runtime({
+        .fixedDeltaTime = 0.25,
+        .maxCatchUpSteps = 4,
+        .baseSeed = 1,
+    });
+
+    runtime.addSystem(std::make_unique<ConfigureSpawnMarkerSystem>());
+    runtime.addSystem(std::make_unique<ConfigureObserveMarkerSystem>(configuredMarkerCount));
+
+    runtime.play();
+
+    SC_EXPECT_EQ(configuredMarkerCount, std::size_t{1});
+    SC_EXPECT_EQ(runtime.world().query().view<Marker>().size(), std::size_t{1});
+}
+
+SC_TEST(EngineRuntime_Stop_ClearsWorldAndPendingCommandsBeforeNextRun) {
+    safecrowd::engine::EngineRuntime runtime({
+        .fixedDeltaTime = 0.25,
+        .maxCatchUpSteps = 4,
+        .baseSeed = 1,
+    });
+
+    runtime.addSystem(std::make_unique<SpawnMarkerSystem>());
+    runtime.play();
+    runtime.stepFrame(0.25);
+    SC_EXPECT_EQ(runtime.world().query().view<Marker>().size(), std::size_t{1});
+
+    runtime.world().commands().spawnEntity(Marker{});
+    runtime.stop();
+
+    SC_EXPECT_EQ(runtime.world().query().view<Marker>().size(), std::size_t{0});
+
+    runtime.play();
+    runtime.stepFrame(0.25);
+
+    SC_EXPECT_EQ(runtime.world().query().view<Marker>().size(), std::size_t{1});
+}
+
+SC_TEST(EngineRuntime_PausedRuntime_DoesNotAdvanceSimulation) {
+    int count = 0;
+
+    safecrowd::engine::EngineRuntime runtime({
+        .fixedDeltaTime = 0.25,
+        .maxCatchUpSteps = 4,
+        .baseSeed = 1,
+    });
+
+    runtime.addSystem(std::make_unique<UpdateCounterSystem>(count));
+    runtime.play();
+    runtime.stepFrame(0.25);
+    runtime.pause();
+    runtime.stepFrame(1.00);
+
+    const auto& pausedStats = runtime.stats();
+    SC_EXPECT_EQ(count, 1);
+    SC_EXPECT_EQ(pausedStats.frameIndex, 1ULL);
+    SC_EXPECT_EQ(pausedStats.fixedStepIndex, 1ULL);
+    SC_EXPECT_EQ(pausedStats.fixedStepsThisFrame, 0U);
+
+    runtime.play();
+    runtime.stepFrame(0.25);
+    SC_EXPECT_EQ(count, 2);
+}
+
 SC_TEST(EngineRuntimePauseAndStopResetLifecycleState) {
     safecrowd::engine::EngineRuntime runtime({
         .fixedDeltaTime = 0.25,