diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..c18dd8d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+__pycache__/
diff --git a/README.md b/README.md
index 1e8a364..c2eb46c 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,37 @@ uv sync
 uv run bowser
 ```
 
+### Testing
+Run the test suite:
+```bash
+# Install dev dependencies
+uv sync --extra dev
+
+# Run all tests
+uv run pytest
+
+# Run with verbose output
+uv run pytest -v
+
+# Run with coverage report
+uv run pytest --cov=src --cov-report=html
+
+# Run specific test file
+uv run pytest tests/test_browser.py
+```
+
+### Development
+```bash
+# Format code
+uv run black src tests
+
+# Lint code
+uv run ruff check src tests
+
+# Type check
+uv run mypy src
+```
+
 ## Project Structure
 
 ```
diff --git a/pyproject.toml b/pyproject.toml
index 058cf1c..db37c9a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -47,3 +47,29 @@ python_version = "3.11"
 warn_return_any = true
 warn_unused_configs = true
 disallow_untyped_defs = false
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+python_files = ["test_*.py"]
+python_classes = ["Test*"]
+python_functions = ["test_*"]
+addopts = [
+    "-ra",
+    "--strict-markers",
+    "--strict-config",
+    "--showlocals",
+]
+
+[tool.coverage.run]
+source = ["src"]
+omit = ["tests/*", "*/conftest.py"]
+
+[tool.coverage.report]
+exclude_lines = [
+    "pragma: no cover",
+    "def __repr__",
+    "raise AssertionError",
+    "raise NotImplementedError",
+    "if __name__ == .__main__.:",
+    "if TYPE_CHECKING:",
+]
diff --git a/src/browser/__pycache__/browser.cpython-313.pyc b/src/browser/__pycache__/browser.cpython-313.pyc
index b1371b6..345016b 100644
Binary files a/src/browser/__pycache__/browser.cpython-313.pyc and b/src/browser/__pycache__/browser.cpython-313.pyc differ
diff --git a/src/browser/__pycache__/chrome.cpython-313.pyc b/src/browser/__pycache__/chrome.cpython-313.pyc
index 373e54f..00bccf2 100644
Binary files a/src/browser/__pycache__/chrome.cpython-313.pyc and b/src/browser/__pycache__/chrome.cpython-313.pyc differ
diff --git a/src/browser/__pycache__/tab.cpython-313.pyc b/src/browser/__pycache__/tab.cpython-313.pyc
index b758d24..d86817a 100644
Binary files a/src/browser/__pycache__/tab.cpython-313.pyc and b/src/browser/__pycache__/tab.cpython-313.pyc differ
diff --git a/tests/README.md b/tests/README.md
new file mode 100644
index 0000000..2bce8dc
--- /dev/null
+++ b/tests/README.md
@@ -0,0 +1,64 @@
+# Bowser Test Suite
+
+This directory contains the test suite for the Bowser browser.
+
+## Running Tests
+
+Run all tests:
+```bash
+uv run pytest
+```
+
+Run with verbose output:
+```bash
+uv run pytest -v
+```
+
+Run specific test file:
+```bash
+uv run pytest tests/test_browser.py
+```
+
+Run with coverage:
+```bash
+uv run pytest --cov=src --cov-report=html
+```
+
+View coverage report:
+```bash
+open htmlcov/index.html
+```
+
+## Test Organization
+
+- `test_url.py` - URL parsing and resolution
+- `test_parser.py` - HTML/CSS parsing
+- `test_browser.py` - Browser and tab management
+- `test_cookies.py` - Cookie jar functionality
+- `test_layout.py` - Layout engine components
+- `test_render.py` - Rendering primitives
+- `conftest.py` - Shared fixtures and configuration
+
+## Writing Tests
+
+Tests use pytest. Example:
+
+```python
+def test_feature():
+    # Arrange
+    obj = MyClass()
+    
+    # Act
+    result = obj.method()
+    
+    # Assert
+    assert result == expected
+```
+
+Use mocks for GTK components:
+```python
+@patch('src.browser.browser.Gtk')
+def test_with_gtk(mock_gtk):
+    browser = Browser()
+    # test code
+```
diff --git a/tests/conftest.py b/tests/conftest.py
new file mode 100644
index 0000000..086a01d
--- /dev/null
+++ b/tests/conftest.py
@@ -0,0 +1,22 @@
+"""Pytest configuration and fixtures."""
+
+import pytest
+import logging
+
+
+@pytest.fixture(autouse=True)
+def configure_logging():
+    """Configure logging for tests."""
+    logging.basicConfig(
+        level=logging.WARNING,  # Only show warnings/errors in tests
+        format="%(name)s %(levelname)s: %(message)s",
+    )
+
+
+@pytest.fixture
+def mock_browser():
+    """Create a mock browser for testing."""
+    from unittest.mock import Mock
+    browser = Mock()
+    browser._log = Mock()
+    return browser
diff --git a/tests/test_browser.py b/tests/test_browser.py
new file mode 100644
index 0000000..863f957
--- /dev/null
+++ b/tests/test_browser.py
@@ -0,0 +1,210 @@
+"""Tests for browser tab management."""
+
+import pytest
+from unittest.mock import Mock, patch
+from src.browser.browser import Browser
+from src.browser.tab import Tab
+from src.network.url import URL
+
+
+class TestTab:
+    def test_tab_creation(self):
+        browser = Mock()
+        tab = Tab(browser)
+        assert tab.browser is browser
+        assert tab.current_url is None
+        assert tab.history == []
+        assert tab.history_index == -1
+        
+    def test_tab_title_new(self):
+        browser = Mock()
+        tab = Tab(browser)
+        assert tab.title == "New Tab"
+        
+    def test_tab_title_with_url(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        tab.load(URL("https://example.com"))
+        assert "example.com" in tab.title
+        
+    def test_tab_load_adds_history(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        url1 = URL("https://example.com")
+        url2 = URL("https://other.com")
+        
+        tab.load(url1)
+        assert len(tab.history) == 1
+        assert tab.history_index == 0
+        
+        tab.load(url2)
+        assert len(tab.history) == 2
+        assert tab.history_index == 1
+        
+    def test_tab_go_back(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        url1 = URL("https://example.com")
+        url2 = URL("https://other.com")
+        
+        tab.load(url1)
+        tab.load(url2)
+        
+        result = tab.go_back()
+        assert result is True
+        assert tab.history_index == 0
+        
+    def test_tab_go_back_at_start(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        tab.load(URL("https://example.com"))
+        
+        result = tab.go_back()
+        assert result is False
+        assert tab.history_index == 0
+        
+    def test_tab_go_forward(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        
+        tab.load(URL("https://example.com"))
+        tab.load(URL("https://other.com"))
+        tab.go_back()
+        
+        result = tab.go_forward()
+        assert result is True
+        assert tab.history_index == 1
+        
+    def test_tab_go_forward_at_end(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        tab.load(URL("https://example.com"))
+        
+        result = tab.go_forward()
+        assert result is False
+        
+    def test_tab_reload(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        tab.load(URL("https://example.com"))
+        
+        result = tab.reload()
+        assert result is True
+        assert tab.history_index == 0
+        
+    def test_tab_history_truncation(self):
+        browser = Mock()
+        browser._log = Mock()
+        tab = Tab(browser)
+        
+        tab.load(URL("https://example.com"))
+        tab.load(URL("https://other.com"))
+        tab.load(URL("https://third.com"))
+        tab.go_back()  # now at other.com
+        tab.load(URL("https://new.com"))  # should truncate third.com
+        
+        assert len(tab.history) == 3
+        assert tab.history_index == 2
+
+
+@patch('src.browser.browser.Gtk')
+class TestBrowser:
+    def test_browser_creation(self, mock_gtk):
+        browser = Browser()
+        assert browser.tabs == []
+        assert browser.active_tab is None
+        
+    def test_new_tab(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        
+        tab = browser.new_tab("https://example.com")
+        
+        assert len(browser.tabs) == 1
+        assert browser.active_tab is tab
+        assert tab in browser.tabs
+        
+    def test_set_active_tab(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        browser.chrome.paint = Mock()
+        browser.chrome.tabs_box = Mock()
+        
+        tab1 = browser.new_tab("https://example.com")
+        tab2 = browser.new_tab("https://other.com")
+        
+        browser.set_active_tab(tab1)
+        assert browser.active_tab is tab1
+        
+    def test_close_tab(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        browser.chrome.paint = Mock()
+        browser.chrome.tabs_box = Mock()
+        
+        tab1 = browser.new_tab("https://example.com")
+        tab2 = browser.new_tab("https://other.com")
+        
+        browser.close_tab(tab1)
+        
+        assert len(browser.tabs) == 1
+        assert tab1 not in browser.tabs
+        assert browser.active_tab is tab2
+        
+    def test_close_active_tab_selects_previous(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        browser.chrome.paint = Mock()
+        browser.chrome.tabs_box = Mock()
+        
+        tab1 = browser.new_tab("https://example.com")
+        tab2 = browser.new_tab("https://other.com")
+        tab3 = browser.new_tab("https://third.com")
+        
+        browser.close_tab(tab3)
+        assert browser.active_tab is tab2
+        
+    def test_close_last_tab(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        browser.chrome.paint = Mock()
+        browser.chrome.tabs_box = Mock()
+        
+        tab = browser.new_tab("https://example.com")
+        browser.close_tab(tab)
+        
+        assert len(browser.tabs) == 0
+        assert browser.active_tab is None
+        
+    def test_navigate_to(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        browser.chrome.paint = Mock()
+        
+        tab = browser.new_tab("https://example.com")
+        browser.navigate_to("https://other.com")
+        
+        assert len(tab.history) == 2
+        
+    def test_navigate_to_no_active_tab(self, mock_gtk):
+        browser = Browser()
+        browser.chrome.rebuild_tab_bar = Mock()
+        browser.chrome.update_address_bar = Mock()
+        
+        browser.navigate_to("https://example.com")
+        
+        assert len(browser.tabs) == 1
+        assert browser.active_tab is not None
diff --git a/tests/test_cookies.py b/tests/test_cookies.py
new file mode 100644
index 0000000..eb4bda0
--- /dev/null
+++ b/tests/test_cookies.py
@@ -0,0 +1,41 @@
+"""Tests for cookie management."""
+
+import pytest
+from src.network.cookies import CookieJar
+
+
+class TestCookieJar:
+    def test_cookie_jar_creation(self):
+        jar = CookieJar()
+        assert jar._cookies == {}
+        
+    def test_set_cookies(self):
+        jar = CookieJar()
+        jar.set_cookies("https://example.com", "session=abc123")
+        
+        cookies = jar.get_cookie_header("https://example.com")
+        assert "session=abc123" in cookies
+        
+    def test_get_cookie_header_empty(self):
+        jar = CookieJar()
+        cookies = jar.get_cookie_header("https://example.com")
+        assert cookies == ""
+        
+    def test_multiple_cookies_same_origin(self):
+        jar = CookieJar()
+        jar.set_cookies("https://example.com", "session=abc123")
+        jar.set_cookies("https://example.com", "user=john")
+        
+        cookies = jar.get_cookie_header("https://example.com")
+        assert "session=abc123" in cookies or "user=john" in cookies
+        
+    def test_cookies_isolated_by_origin(self):
+        jar = CookieJar()
+        jar.set_cookies("https://example.com", "session=abc123")
+        jar.set_cookies("https://other.com", "session=xyz789")
+        
+        cookies1 = jar.get_cookie_header("https://example.com")
+        cookies2 = jar.get_cookie_header("https://other.com")
+        
+        assert "abc123" in cookies1
+        assert "xyz789" in cookies2
diff --git a/tests/test_layout.py b/tests/test_layout.py
new file mode 100644
index 0000000..4960115
--- /dev/null
+++ b/tests/test_layout.py
@@ -0,0 +1,58 @@
+"""Tests for layout components."""
+
+import pytest
+from unittest.mock import Mock
+from src.layout.document import DocumentLayout
+from src.layout.block import BlockLayout, LineLayout
+from src.layout.inline import TextLayout
+from src.parser.html import Element, Text
+
+
+class TestDocumentLayout:
+    def test_document_layout_creation(self):
+        node = Element("html")
+        layout = DocumentLayout(node)
+        assert layout.node is node
+        assert layout.children == []
+        
+    def test_document_layout(self):
+        node = Element("html")
+        layout = DocumentLayout(node)
+        result = layout.layout(800, 1.0)
+        assert result == 800.0
+
+
+class TestBlockLayout:
+    def test_block_layout_creation(self):
+        node = Element("div")
+        layout = BlockLayout(node)
+        assert layout.node is node
+        assert layout.children == []
+        
+    def test_block_layout_with_parent(self):
+        parent_node = Element("body")
+        child_node = Element("div")
+        parent_layout = BlockLayout(parent_node)
+        child_layout = BlockLayout(child_node, parent=parent_layout)
+        assert child_layout.parent is parent_layout
+
+
+class TestLineLayout:
+    def test_line_layout_creation(self):
+        node = Element("span")
+        layout = LineLayout(node)
+        assert layout.node is node
+
+
+class TestTextLayout:
+    def test_text_layout_creation(self):
+        node = Text("Hello")
+        layout = TextLayout(node, "Hello")
+        assert layout.node is node
+        assert layout.word == "Hello"
+        
+    def test_text_layout_length(self):
+        node = Text("Hello")
+        layout = TextLayout(node, "Hello")
+        result = layout.layout()
+        assert result == 5
diff --git a/tests/test_parser.py b/tests/test_parser.py
new file mode 100644
index 0000000..c071e91
--- /dev/null
+++ b/tests/test_parser.py
@@ -0,0 +1,52 @@
+"""Tests for HTML parsing."""
+
+import pytest
+from src.parser.html import Text, Element, print_tree
+
+
+class TestHTMLElements:
+    def test_text_node(self):
+        text = Text("Hello World")
+        assert text.text == "Hello World"
+        assert text.parent is None
+        
+    def test_text_node_with_parent(self):
+        parent = Element("div")
+        text = Text("Hello", parent=parent)
+        assert text.parent is parent
+        
+    def test_element_node(self):
+        elem = Element("div", {"class": "container"})
+        assert elem.tag == "div"
+        assert elem.attributes == {"class": "container"}
+        assert elem.children == []
+        
+    def test_element_default_attributes(self):
+        elem = Element("p")
+        assert elem.attributes == {}
+        
+    def test_element_parent(self):
+        parent = Element("body")
+        child = Element("div", parent=parent)
+        assert child.parent is parent
+
+
+class TestPrintTree:
+    def test_print_single_element(self, capsys):
+        elem = Element("div")
+        print_tree(elem)
+        captured = capsys.readouterr()
+        assert "Element('div'" in captured.out
+        
+    def test_print_tree_with_children(self, capsys):
+        root = Element("html")
+        body = Element("body", parent=root)
+        text = Text("Hello", parent=body)
+        root.children = [body]
+        body.children = [text]
+        
+        print_tree(root)
+        captured = capsys.readouterr()
+        assert "Element('html'" in captured.out
+        assert "Element('body'" in captured.out
+        assert "Text('Hello')" in captured.out
diff --git a/tests/test_render.py b/tests/test_render.py
new file mode 100644
index 0000000..e59e515
--- /dev/null
+++ b/tests/test_render.py
@@ -0,0 +1,55 @@
+"""Tests for rendering primitives."""
+
+import pytest
+from src.render.paint import PaintCommand, DrawText
+from src.render.composite import CompositedLayer
+from src.render.fonts import get_font, linespace
+
+
+class TestPaintCommands:
+    def test_paint_command_creation(self):
+        cmd = PaintCommand((0, 0, 100, 100))
+        assert cmd.rect == (0, 0, 100, 100)
+        
+    def test_draw_text_creation(self):
+        cmd = DrawText(10, 20, "Hello", ("Arial", 12), "black")
+        assert cmd.text == "Hello"
+        assert cmd.font == ("Arial", 12)
+        assert cmd.color == "black"
+
+
+class TestCompositedLayer:
+    def test_composited_layer_creation(self):
+        layer = CompositedLayer()
+        assert layer.items == []
+        
+    def test_composited_layer_with_item(self):
+        item = "mock_item"
+        layer = CompositedLayer(item)
+        assert len(layer.items) == 1
+        assert layer.items[0] == item
+        
+    def test_add_item(self):
+        layer = CompositedLayer()
+        layer.add("item1")
+        layer.add("item2")
+        assert len(layer.items) == 2
+
+
+class TestFonts:
+    def test_get_font(self):
+        font = get_font(14)
+        assert font == (14, "normal", "normal")
+        
+    def test_get_font_with_weight(self):
+        font = get_font(16, weight="bold")
+        assert font == (16, "bold", "normal")
+        
+    def test_get_font_with_style(self):
+        font = get_font(12, style="italic")
+        assert font == (12, "normal", "italic")
+        
+    def test_linespace(self):
+        font = (14, "normal", "normal")
+        space = linespace(font)
+        assert space == int(14 * 1.2)
diff --git a/tests/test_url.py b/tests/test_url.py
new file mode 100644
index 0000000..fd0dc95
--- /dev/null
+++ b/tests/test_url.py
@@ -0,0 +1,41 @@
+"""Tests for URL parsing and resolution."""
+
+import pytest
+from src.network.url import URL
+
+
+class TestURL:
+    def test_parse_simple_url(self):
+        url = URL("https://example.com")
+        assert str(url) == "https://example.com"
+        
+    def test_parse_url_with_path(self):
+        url = URL("https://example.com/path/to/page")
+        assert str(url) == "https://example.com/path/to/page"
+        
+    def test_parse_url_with_query(self):
+        url = URL("https://example.com/search?q=test")
+        assert str(url) == "https://example.com/search?q=test"
+        
+    def test_origin(self):
+        url = URL("https://example.com:8080/path")
+        assert url.origin() == "https://example.com:8080"
+        
+    def test_origin_default_port(self):
+        url = URL("https://example.com/path")
+        assert url.origin() == "https://example.com"
+        
+    def test_resolve_relative_path(self):
+        base = URL("https://example.com/dir/page.html")
+        resolved = base.resolve("other.html")
+        assert str(resolved) == "https://example.com/dir/other.html"
+        
+    def test_resolve_absolute_path(self):
+        base = URL("https://example.com/dir/page.html")
+        resolved = base.resolve("/root/page.html")
+        assert str(resolved) == "https://example.com/root/page.html"
+        
+    def test_resolve_full_url(self):
+        base = URL("https://example.com/page.html")
+        resolved = base.resolve("https://other.com/page.html")
+        assert str(resolved) == "https://other.com/page.html"