diff --git a/.codecov.yml b/.codecov.yml
new file mode 100644
index 0000000..c1c3711
--- /dev/null
+++ b/.codecov.yml
@@ -0,0 +1,3 @@
+ignore:
+  - catch.hpp
+  - catch_main.hpp
diff --git a/.travis.yml b/.travis.yml
index ffa348e..18f7714 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -62,6 +62,8 @@ matrix:
     - os: osx
       osx_image: xcode10
       compiler: clang
+      addons: { homebrew: { packages: ["lcov"] } }
+      after_success: ./scripts/upload_coverage.sh
 before_install:
   - eval "${MATRIX_EVAL}"
 script:
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4adf7e5..2f6875c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,6 +1,36 @@
 cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
 project(ecs)
 
+#
+# coverage mode
+#
+
+option(ECS_BUILD_WITH_COVERAGE "Build with coverage" OFF)
+if(ECS_BUILD_WITH_COVERAGE AND (CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang"))
+    add_definitions(-DECS_BUILD_WITH_COVERAGE)
+    set(ECS_COVERAGE_FLAGS "--coverage")
+    set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} ${ECS_COVERAGE_FLAGS}")
+    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} ${ECS_COVERAGE_FLAGS}")
+    set(CMAKE_LINKER_FLAGS_DEBUG "${CMAKE_LINKER_FLAGS_DEBUG} ${ECS_COVERAGE_FLAGS}")
+endif()
+
+#
+# sanitizer mode
+#
+
+option(ECS_BUILD_WITH_SANITIZER "Build with sanitizer" OFF)
+if(ECS_BUILD_WITH_SANITIZER AND (CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang"))
+    add_definitions(-DECS_BUILD_WITH_SANITIZER)
+    set(ECS_SANITIZER_FLAGS "-fno-omit-frame-pointer -fsanitize=address")
+    set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} ${ECS_SANITIZER_FLAGS}")
+    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} ${ECS_SANITIZER_FLAGS}")
+    set(CMAKE_LINKER_FLAGS_DEBUG "${CMAKE_LINKER_FLAGS_DEBUG} ${ECS_SANITIZER_FLAGS}")
+endif()
+
+#
+# tests executable
+#
+
 file(GLOB test_sources "*.cpp" "*.hpp")
 add_executable(${PROJECT_NAME} ${test_sources})
 
diff --git a/README.md b/README.md
index eba976e..7867a5d 100644
--- a/README.md
+++ b/README.md
@@ -2,18 +2,21 @@
 
 [![travis][badge.travis]][travis]
 [![appveyor][badge.appveyor]][appveyor]
+[![codecov][badge.codecov]][codecov]
 [![language][badge.language]][language]
 [![license][badge.license]][license]
 [![paypal][badge.paypal]][paypal]
 
-[badge.travis]: https://img.shields.io/travis/BlackMATov/ecs.hpp/master.svg?logo=travis&style=for-the-badge
-[badge.appveyor]: https://img.shields.io/appveyor/ci/BlackMATov/ecs-hpp/master.svg?logo=appveyor&style=for-the-badge
-[badge.language]: https://img.shields.io/badge/language-C%2B%2B14-red.svg?style=for-the-badge
-[badge.license]: https://img.shields.io/badge/license-MIT-blue.svg?style=for-the-badge
-[badge.paypal]: https://img.shields.io/badge/donate-PayPal-orange.svg?logo=paypal&colorA=00457C&style=for-the-badge
+[badge.travis]: https://img.shields.io/travis/BlackMATov/ecs.hpp/master.svg?logo=travis
+[badge.appveyor]: https://img.shields.io/appveyor/ci/BlackMATov/ecs-hpp/master.svg?logo=appveyor
+[badge.codecov]: https://img.shields.io/codecov/c/github/BlackMATov/ecs-hpp/master.svg?logo=codecov
+[badge.language]: https://img.shields.io/badge/language-C%2B%2B14-red.svg
+[badge.license]: https://img.shields.io/badge/license-MIT-blue.svg
+[badge.paypal]: https://img.shields.io/badge/donate-PayPal-orange.svg?logo=paypal&colorA=00457C
 
 [travis]: https://travis-ci.org/BlackMATov/ecs.hpp
 [appveyor]: https://ci.appveyor.com/project/BlackMATov/ecs-hpp
+[codecov]: https://codecov.io/gh/BlackMATov/ecs.hpp
 [language]: https://en.wikipedia.org/wiki/C%2B%2B14
 [license]: https://en.wikipedia.org/wiki/MIT_License
 [paypal]: https://www.paypal.me/matov
diff --git a/ecs.hpp b/ecs.hpp
index 1409a9e..60e5e16 100644
--- a/ecs.hpp
+++ b/ecs.hpp
@@ -6,6 +6,151 @@
 
 #pragma once
 
+#include <cassert>
+#include <cstdint>
+
+#include <mutex>
+#include <limits>
+#include <functional>
+#include <unordered_set>
+
+// -----------------------------------------------------------------------------
+//
+// config
+//
+// -----------------------------------------------------------------------------
+
 namespace ecs_hpp
 {
+    class world;
+    class entity;
+
+    using entity_id = std::uint64_t;
+}
+
+// -----------------------------------------------------------------------------
+//
+// entity
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    class entity final {
+    public:
+        entity(world& owner);
+        entity(world& owner, entity_id id);
+
+        const world& owner() const noexcept;
+        entity_id id() const noexcept;
+
+        bool destroy();
+    private:
+        world& owner_;
+        entity_id id_{0u};
+    };
+
+    bool operator==(const entity& l, const entity& r) noexcept;
+    bool operator!=(const entity& l, const entity& r) noexcept;
+}
+
+namespace std
+{
+    template <>
+    struct hash<ecs_hpp::entity>
+        : std::unary_function<const ecs_hpp::entity&, std::size_t>
+    {
+        std::size_t operator()(const ecs_hpp::entity& ent) const noexcept {
+            return std::hash<ecs_hpp::entity_id>()(ent.id());
+        }
+    };
+}
+
+// -----------------------------------------------------------------------------
+//
+// world
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    class world final {
+    public:
+        world();
+        ~world() noexcept;
+
+        entity create_entity();
+        bool destroy_entity(const entity& ent);
+        bool is_entity_alive(const entity& ent) const noexcept;
+    private:
+        mutable std::mutex mutex_;
+        entity_id last_entity_id_{0u};
+        std::unordered_set<entity> entities_;
+    };
+}
+
+// -----------------------------------------------------------------------------
+//
+// entity impl
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    inline entity::entity(world& owner)
+    : owner_(owner) {}
+
+    inline entity::entity(world& owner, entity_id id)
+    : owner_(owner)
+    , id_(id) {}
+
+    inline const world& entity::owner() const noexcept {
+        return owner_;
+    }
+
+    inline entity_id entity::id() const noexcept {
+        return id_;
+    }
+
+    inline bool entity::destroy() {
+        return owner_.destroy_entity(*this);
+    }
+
+    inline bool operator==(const entity& l, const entity& r) noexcept {
+        return l.id() == r.id();
+    }
+
+    inline bool operator!=(const entity& l, const entity& r) noexcept {
+        return !(l == r);
+    }
+}
+
+// -----------------------------------------------------------------------------
+//
+// world impl
+//
+// -----------------------------------------------------------------------------
+
+namespace ecs_hpp
+{
+    inline world::world() = default;
+    inline world::~world() noexcept = default;
+
+    inline entity world::create_entity() {
+        std::lock_guard<std::mutex> guard(mutex_);
+        assert(last_entity_id_ < std::numeric_limits<entity_id>::max());
+        auto ent = entity(*this, ++last_entity_id_);
+        entities_.insert(ent);
+        return ent;
+    }
+
+    inline bool world::destroy_entity(const entity& ent) {
+        std::lock_guard<std::mutex> guard(mutex_);
+        return entities_.erase(ent) > 0u;
+    }
+
+    inline bool world::is_entity_alive(const entity& ent) const noexcept {
+        std::lock_guard<std::mutex> guard(mutex_);
+        return entities_.count(ent) > 0u;
+    }
 }
diff --git a/ecs_tests.cpp b/ecs_tests.cpp
index 584701f..57bb26a 100644
--- a/ecs_tests.cpp
+++ b/ecs_tests.cpp
@@ -12,7 +12,60 @@ namespace ecs = ecs_hpp;
 
 namespace
 {
+    struct position {
+        int x{0};
+        int y{0};
+
+        position() = default;
+        position(int nx, int ny) : x(nx), y(ny) {}
+    };
+
+    struct velocity {
+        int dx{0};
+        int dy{0};
+
+        velocity() = default;
+        velocity(int ndx, int ndy) : dx(ndx), dy(ndy) {}
+    };
 }
 
-TEST_CASE("ecs") {
+TEST_CASE("world") {
+    SECTION("entities") {
+        {
+            ecs::world w;
+
+            ecs::entity e1{w};
+            ecs::entity e2{w};
+
+            REQUIRE(e1 == e2);
+            REQUIRE_FALSE(w.is_entity_alive(e1));
+            REQUIRE_FALSE(w.is_entity_alive(e2));
+
+            REQUIRE_FALSE(w.destroy_entity(e1));
+            REQUIRE_FALSE(w.destroy_entity(e2));
+        }
+        {
+            ecs::world w;
+
+            auto e1 = w.create_entity();
+            auto e2 = w.create_entity();
+
+            REQUIRE(e1 != e2);
+            REQUIRE(w.is_entity_alive(e1));
+            REQUIRE(w.is_entity_alive(e2));
+
+            REQUIRE(w.destroy_entity(e1));
+            REQUIRE_FALSE(w.is_entity_alive(e1));
+            REQUIRE(w.is_entity_alive(e2));
+
+            REQUIRE(w.destroy_entity(e2));
+            REQUIRE_FALSE(w.is_entity_alive(e1));
+            REQUIRE_FALSE(w.is_entity_alive(e2));
+
+            REQUIRE_FALSE(w.destroy_entity(e1));
+            REQUIRE_FALSE(w.destroy_entity(e2));
+        }
+    }
+    SECTION("components") {
+    }
 }
diff --git a/scripts/upload_coverage.sh b/scripts/upload_coverage.sh
new file mode 100755
index 0000000..d13a427
--- /dev/null
+++ b/scripts/upload_coverage.sh
@@ -0,0 +1,17 @@
+#!/bin/bash
+set -e
+
+BUILD_DIR=`dirname "$BASH_SOURCE"`/../build
+mkdir -p $BUILD_DIR/coverage
+cd $BUILD_DIR/coverage
+cmake -DCMAKE_BUILD_TYPE=Debug -DECS_BUILD_WITH_COVERAGE=ON ../..
+cmake --build . -- -j8
+
+lcov -d . -z
+ctest --verbose
+
+lcov -d . -c -o "coverage.info"
+lcov -r "coverage.info" "*/usr/*" "*/catch.hpp" "*/catch_main.cpp" -o "coverage.info"
+lcov -l "coverage.info"
+
+bash <(curl -s https://codecov.io/bash) || echo "Codecov did not collect coverage reports"