diff --git a/tests/unit/test_complexity.py b/tests/unit/test_complexity.py
new file mode 100644
index 0000000..f1c7796
--- /dev/null
+++ b/tests/unit/test_complexity.py
@@ -0,0 +1,270 @@
+"""Unit tests for complexity analysis module."""
+
+import pytest
+from codesnap.core.complexity import (
+    calculate_cyclomatic_complexity,
+    calculate_nesting_depth,
+    count_lines,
+    rate_complexity,
+    analyze_file_complexity,
+    get_complexity_summary,
+    ComplexityMetrics,
+)
+from codesnap.core.parser import FunctionInfo
+
+
+class TestCalculateCyclomaticComplexity:
+    """Tests for cyclomatic complexity calculation."""
+    
+    def test_empty_content(self):
+        complexity, decisions = calculate_cyclomatic_complexity("")
+        assert complexity == 1
+        assert decisions == 0
+    
+    def test_simple_function(self):
+        content = "def test():\n    pass"
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity == 1
+    
+    def test_if_statement(self):
+        content = "if x > 0:\n    pass"
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 1
+    
+    def test_multiple_if_statements(self):
+        content = """
+if x > 0:
+    pass
+elif x < 0:
+    pass
+else:
+    pass
+"""
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 3
+    
+    def test_for_loop(self):
+        content = "for i in range(10):\n    pass"
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 1
+    
+    def test_while_loop(self):
+        content = "while True:\n    pass"
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 1
+    
+    def test_try_except(self):
+        content = """
+try:
+    pass
+except Exception:
+    pass
+"""
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 1
+    
+    def test_and_or_operators(self):
+        content = "if x > 0 and y > 0:\n    pass"
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 2
+    
+    def test_ternary_operator(self):
+        content = "x = 1 if cond else 2"
+        complexity, decisions = calculate_cyclomatic_complexity(content)
+        assert complexity >= 1
+
+
+class TestCalculateNestingDepth:
+    """Tests for nesting depth calculation."""
+    
+    def test_flat_code(self):
+        depth = calculate_nesting_depth("x = 1\ny = 2")
+        assert depth >= 0
+    
+    def test_single_brace_level(self):
+        depth = calculate_nesting_depth("if x: { y = 1 }")
+        assert depth >= 0
+    
+    def test_nested_braces(self):
+        content = """
+if x:
+    if y:
+        if z:
+            pass
+"""
+        depth = calculate_nesting_depth(content)
+        assert depth >= 0
+    
+    def test_mixed_brackets(self):
+        content = """
+def test():
+    data = [
+        [1, 2],
+        {a: b}
+    ]
+"""
+        depth = calculate_nesting_depth(content)
+        assert depth >= 1
+    
+    def test_balanced_brackets(self):
+        content = "[](){}"
+        depth = calculate_nesting_depth(content)
+        assert depth >= 1
+    
+    def test_unbalanced_close(self):
+        content = "x = 1]"
+        depth = calculate_nesting_depth(content)
+        assert depth >= 0
+
+
+class TestCountLines:
+    """Tests for line counting."""
+    
+    def test_empty_content(self):
+        total, comments = count_lines("")
+        assert total >= 0
+        assert comments >= 0
+    
+    def test_single_line(self):
+        total, comments = count_lines("x = 1")
+        assert total >= 1
+        assert comments >= 0
+    
+    def test_python_comments(self):
+        content = "# This is a comment\nx = 1\n# Another comment"
+        total, comments = count_lines(content)
+        assert total >= 3
+        assert comments >= 2
+    
+    def test_python_docstring(self):
+        content = '"""This is a docstring"""'
+        total, comments = count_lines(content)
+        assert total >= 1
+    
+    def test_multiline_python_comment(self):
+        content = """
+'''
+Multiline
+Comment
+'''
+x = 1
+"""
+        total, comments = count_lines(content)
+        assert total >= 5
+    
+    def test_cpp_comments(self):
+        content = "// Single line comment\nx = 1;"
+        total, comments = count_lines(content)
+        assert total >= 2
+        assert comments >= 1
+    
+    def test_c_multiline_comment(self):
+        content = "/* Multi\n   Line */\nx = 1;"
+        total, comments = count_lines(content)
+        assert total >= 3
+        assert comments >= 1
+
+
+class TestRateComplexity:
+    """Tests for complexity rating."""
+    
+    def test_low_complexity(self):
+        assert rate_complexity(1, 1) == "low"
+        assert rate_complexity(5, 2) == "low"
+        assert rate_complexity(9, 3) == "low"
+    
+    def test_medium_complexity(self):
+        assert rate_complexity(10, 3) == "medium"
+        assert rate_complexity(15, 4) == "medium"
+        assert rate_complexity(19, 5) == "medium"
+    
+    def test_high_complexity(self):
+        assert rate_complexity(20, 3) == "high"
+        assert rate_complexity(25, 6) == "high"
+        assert rate_complexity(50, 2) == "high"
+    
+    def test_high_nesting(self):
+        result = rate_complexity(5, 6)
+        assert result in ["low", "medium", "high"]
+
+
+class TestAnalyzeFileComplexity:
+    """Tests for file complexity analysis."""
+    
+    def test_empty_file(self):
+        metrics, func_complexities = analyze_file_complexity("", [], "python")
+        assert metrics.cyclomatic_complexity >= 1
+        assert len(func_complexities) == 0
+    
+    def test_simple_file(self):
+        content = "x = 1\ny = 2"
+        metrics, func_complexities = analyze_file_complexity(content, [], "python")
+        assert metrics.complexity_rating in ["low", "medium", "high"]
+    
+    def test_complex_file(self):
+        content = """
+def test():
+    if x > 0:
+        if y > 0:
+            if z > 0:
+                pass
+"""
+        func = FunctionInfo(
+            name="test",
+            node_type="function",
+            start_line=1,
+            end_line=6,
+            parameters=[],
+        )
+        metrics, func_complexities = analyze_file_complexity(content, [func], "python")
+        assert metrics.complexity_rating in ["low", "medium", "high"]
+        assert len(func_complexities) >= 0
+    
+    def test_suggestions_generated(self):
+        content = """
+def test():
+    pass
+""" * 25
+        metrics, func_complexities = analyze_file_complexity(content, [], "python")
+        assert isinstance(metrics.suggestions, list)
+
+
+class TestGetComplexitySummary:
+    """Tests for complexity summary generation."""
+    
+    def test_empty_list(self):
+        summary = get_complexity_summary([])
+        assert summary["total_files"] == 0
+        assert summary["avg_complexity"] == 0
+    
+    def test_single_file(self):
+        metrics = ComplexityMetrics(
+            cyclomatic_complexity=10,
+            nesting_depth=2,
+            lines_of_code=50,
+        )
+        summary = get_complexity_summary([metrics])
+        assert summary["total_files"] == 1
+        assert summary["avg_complexity"] == 10
+    
+    def test_multiple_files(self):
+        metrics_list = [
+            ComplexityMetrics(cyclomatic_complexity=5),
+            ComplexityMetrics(cyclomatic_complexity=15),
+            ComplexityMetrics(cyclomatic_complexity=10),
+        ]
+        summary = get_complexity_summary(metrics_list)
+        assert summary["total_files"] == 3
+        assert summary["avg_complexity"] == 10
+    
+    def test_rating_distribution(self):
+        metrics_list = [
+            ComplexityMetrics(cyclomatic_complexity=5),
+            ComplexityMetrics(cyclomatic_complexity=15),
+            ComplexityMetrics(cyclomatic_complexity=25),
+        ]
+        summary = get_complexity_summary(metrics_list)
+        assert summary["rating_distribution"]["low"] >= 0
+        assert summary["rating_distribution"]["medium"] >= 0
+        assert summary["rating_distribution"]["high"] >= 0
+        assert summary["rating_distribution"]["low"] + summary["rating_distribution"]["medium"] + summary["rating_distribution"]["high"] == 3