diff --git a/tests/test_explainer.py b/tests/test_explainer.py
new file mode 100644
index 0000000..dc07b2b
--- /dev/null
+++ b/tests/test_explainer.py
@@ -0,0 +1,127 @@
+"""Tests for the explainer module."""
+
+import pytest
+from cli_explain_fix.parser import ErrorParser, ParsedError
+from cli_explain_fix.explainer import Explainer
+from cli_explain_fix.knowledge_base import KnowledgeBase
+
+
+class TestExplainer:
+    """Test cases for Explainer."""
+
+    def setup_method(self):
+        """Set up explainer instance for each test."""
+        self.parser = ErrorParser()
+        self.kb = KnowledgeBase()
+        self.explainer = Explainer(self.kb)
+
+    def test_explain_python_error(self, sample_python_simple_error):
+        """Test explaining a Python error."""
+        parsed = self.parser.parse(sample_python_simple_error)
+        explanation = self.explainer.explain(parsed)
+
+        assert "error_type" in explanation
+        assert "language" in explanation
+        assert "summary" in explanation
+        assert "what_happened" in explanation
+        assert "why_happened" in explanation
+        assert "how_to_fix" in explanation
+
+        assert explanation["error_type"] == "ValueError"
+        assert explanation["language"] == "python"
+
+    def test_explain_python_traceback(self, sample_python_traceback):
+        """Test explaining Python traceback."""
+        parsed = self.parser.parse(sample_python_traceback)
+        explanation = self.explainer.explain(parsed)
+
+        assert explanation["error_type"] == "ModuleNotFoundError"
+        assert explanation["language"] == "python"
+        assert "location" in explanation
+        assert "/app/main.py" in explanation["location"]["file"]
+        assert explanation["location"]["line"] == 10
+
+    def test_explain_javascript_error(self, sample_js_error):
+        """Test explaining JavaScript error."""
+        parsed = self.parser.parse(sample_js_error)
+        explanation = self.explainer.explain(parsed)
+
+        assert explanation["error_type"] == "TypeError"
+        assert explanation["language"] == "javascript"
+
+    def test_explain_verbose_mode(self, sample_python_simple_error):
+        """Test explaining with verbose flag."""
+        parsed = self.parser.parse(sample_python_simple_error)
+        explanation = self.explainer.explain(parsed, verbose=True)
+
+        assert "raw_error" in explanation
+
+    def test_explain_without_verbose(self, sample_python_simple_error):
+        """Test explaining without verbose flag."""
+        parsed = self.parser.parse(sample_python_simple_error)
+        explanation = self.explainer.explain(parsed, verbose=False)
+
+        assert "raw_error" not in explanation
+
+    def test_explain_with_stack_trace(self, sample_python_traceback):
+        """Test explaining error with stack frames."""
+        parsed = self.parser.parse(sample_python_traceback)
+        explanation = self.explainer.explain(parsed)
+
+        assert "stack_trace" in explanation
+        assert len(explanation["stack_trace"]) > 0
+
+    def test_explain_with_code_examples(self, sample_python_simple_error):
+        """Test that code examples are included."""
+        parsed = self.parser.parse(sample_python_simple_error)
+        explanation = self.explainer.explain(parsed)
+
+        if "code_examples" in explanation:
+            assert isinstance(explanation["code_examples"], list)
+
+    def test_explain_simple(self, sample_python_simple_error):
+        """Test simple text explanation."""
+        result = self.explainer.explain_simple(
+            "ValueError",
+            "invalid value for int()",
+            "python"
+        )
+
+        assert "Error: ValueError" in result
+        assert "Language: python" in result
+        assert "What happened:" in result
+        assert "How to fix:" in result
+
+    def test_get_fix_steps(self, sample_python_simple_error):
+        """Test getting fix steps for an error."""
+        parsed = self.parser.parse(sample_python_simple_error)
+        steps = self.explainer.get_fix_steps(parsed)
+
+        assert isinstance(steps, list)
+        assert len(steps) > 0
+
+    def test_explain_unknown_error(self, sample_unknown_error):
+        """Test explaining an unknown error type."""
+        parsed = self.parser.parse(sample_unknown_error)
+        explanation = self.explainer.explain(parsed)
+
+        assert "error_type" in explanation
+        assert "what_happened" in explanation
+        assert "how_to_fix" in explanation
+
+
+class TestExplainerSummary:
+    """Test cases for explanation summary generation."""
+
+    def test_summary_format(self, sample_python_simple_error):
+        """Test summary format is correct."""
+        parser = ErrorParser()
+        kb = KnowledgeBase()
+        explainer = Explainer(kb)
+
+        parsed = parser.parse(sample_python_simple_error)
+        explanation = explainer.explain(parsed)
+
+        summary = explanation["summary"]
+        assert "ValueError" in summary
+        assert "python" in summary