diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b2207cb..9d96d3a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,6 +21,9 @@ jobs:
 
       - name: Lint
         run: bunx biome check .
+      
+      - name: Test
+        run: bun test
 
       - name: Build
         run: bun run build
\ No newline at end of file
diff --git a/.husky/pre-push b/.husky/pre-push
index e4b1180..ca61e7b 100644
--- a/.husky/pre-push
+++ b/.husky/pre-push
@@ -1 +1,3 @@
-bun run build
\ No newline at end of file
+[ -n "$CI" ] && exit 0
+bun run build
+bun test
\ No newline at end of file
diff --git a/package.json b/package.json
index 94a1b86..088063a 100644
--- a/package.json
+++ b/package.json
@@ -49,7 +49,8 @@
 		"release": "bun run build && changeset publish",
 		"version-packages": "changeset version",
 		"build": "tsup",
-		"check": "biome check --write ."
+		"check": "biome check --write .",
+		"test": "bun test"
 	},
 	"lint-staged": {
 		"*.{ts,tsx,js,jsx,json,css,md}": [
diff --git a/tests/interpreter.test.ts b/tests/interpreter.test.ts
new file mode 100644
index 0000000..12e46ca
--- /dev/null
+++ b/tests/interpreter.test.ts
@@ -0,0 +1,97 @@
+import { describe, expect, spyOn, test } from "bun:test";
+import { Interpreter } from "../src/interpreter";
+import { Lexer } from "../src/lexer";
+import { Parser } from "../src/parser";
+
+let output: string[];
+let logSpy: ReturnType<typeof spyOn>;
+
+function run(source: string): string[] {
+	output = [];
+	logSpy = spyOn(console, "log").mockImplementation((...args: unknown[]) => {
+		output.push(args.map(String).join(" "));
+	});
+
+	const tokens = new Lexer(source).tokenize();
+	const ast = new Parser(tokens).parse();
+	new Interpreter().run(ast);
+
+	logSpy.mockRestore();
+	return output;
+}
+
+describe("Interpreter", () => {
+	test("declares and prints a variable", () => {
+		const result = run("new x = 42;\nprint(x);");
+		expect(result).toEqual(["42"]);
+	});
+
+	test("prints a number literal", () => {
+		const result = run("print(10);");
+		expect(result).toEqual(["10"]);
+	});
+
+	test("evaluates addition", () => {
+		const result = run("new a = 10;\nnew b = 20;\nprint(a + b);");
+		expect(result).toEqual(["30"]);
+	});
+
+	test("evaluates subtraction", () => {
+		const result = run("new a = 50;\nnew b = 8;\nprint(a - b);");
+		expect(result).toEqual(["42"]);
+	});
+
+	test("evaluates chained operations", () => {
+		const result = run("print(10 + 20 - 5);");
+		expect(result).toEqual(["25"]);
+	});
+
+	test("reassigns variables", () => {
+		const result = run("new x = 1;\nx = 99;\nprint(x);");
+		expect(result).toEqual(["99"]);
+	});
+
+	test("reassigns with expression", () => {
+		const result = run("new x = 10;\nx = x + 5;\nprint(x);");
+		expect(result).toEqual(["15"]);
+	});
+
+	test("handles multiple prints", () => {
+		const result = run(
+			"new a = 1;\nnew b = 2;\nnew c = 3;\nprint(a);\nprint(b);\nprint(c);",
+		);
+		expect(result).toEqual(["1", "2", "3"]);
+	});
+
+	test("throws on duplicate declaration", () => {
+		expect(() => run("new x = 1;\nnew x = 2;")).toThrow(
+			"Variable 'x' is already declared",
+		);
+	});
+
+	test("throws on undeclared variable in print", () => {
+		expect(() => run("print(x);")).toThrow("Variable 'x' is not declared");
+	});
+
+	test("throws on assignment to undeclared variable", () => {
+		expect(() => run("x = 10;")).toThrow("Variable 'x' is not declared");
+	});
+
+	test("throws on undeclared variable in expression", () => {
+		expect(() => run("new x = 1;\nprint(x + y);")).toThrow(
+			"Variable 'y' is not declared",
+		);
+	});
+
+	test("full program", () => {
+		const result = run(`
+			new a = 10;
+			new b = 20;
+			new c = a + b;
+			print(c);
+			a = 5;
+			print(a - b);
+		`);
+		expect(result).toEqual(["30", "-15"]);
+	});
+});
diff --git a/tests/lexer.test.ts b/tests/lexer.test.ts
new file mode 100644
index 0000000..ab23991
--- /dev/null
+++ b/tests/lexer.test.ts
@@ -0,0 +1,113 @@
+import { describe, expect, test } from "bun:test";
+import { Lexer, TokenType } from "../src/lexer";
+
+describe("Lexer", () => {
+	test("tokenizes number", () => {
+		const tokens = new Lexer("51").tokenize();
+		expect(tokens[0]).toMatchObject({ type: TokenType.Number, value: "51" });
+		expect(tokens[1]).toMatchObject({ type: TokenType.EOF });
+	});
+
+	test("tokenizes identifier", () => {
+		const tokens = new Lexer("foo").tokenize();
+		expect(tokens[0]).toMatchObject({
+			type: TokenType.Identifier,
+			value: "foo",
+		});
+	});
+
+	test("tokenizes keywords", () => {
+		const tokens = new Lexer("new print").tokenize();
+		expect(tokens[0]).toMatchObject({ type: TokenType.New });
+		expect(tokens[1]).toMatchObject({ type: TokenType.Print });
+	});
+
+	test("tokenizes operators and delimiters", () => {
+		const tokens = new Lexer("= + - ; ( )").tokenize();
+		expect(tokens.map((t) => t.type)).toEqual([
+			TokenType.Equals,
+			TokenType.Plus,
+			TokenType.Minus,
+			TokenType.Semicolon,
+			TokenType.LParen,
+			TokenType.RParen,
+			TokenType.EOF,
+		]);
+	});
+
+	test("tokenizes variable declaration", () => {
+		const tokens = new Lexer("new x = 10;").tokenize();
+		expect(tokens.map((t) => t.type)).toEqual([
+			TokenType.New,
+			TokenType.Identifier,
+			TokenType.Equals,
+			TokenType.Number,
+			TokenType.Semicolon,
+			TokenType.EOF,
+		]);
+	});
+
+	test("skips single-line comments", () => {
+		const tokens = new Lexer("// this is a comment\n42").tokenize();
+		expect(tokens[0]).toMatchObject({ type: TokenType.Number, value: "42" });
+	});
+
+	test("skips inline comments", () => {
+		const tokens = new Lexer("new x = 10; // my var").tokenize();
+		expect(tokens).toHaveLength(6); // new, x, =, 10, ;, EOF
+	});
+
+	test("tracks line numbers", () => {
+		const tokens = new Lexer("new x = 10;\nnew y = 20;").tokenize();
+		const y = tokens.find((t) => t.value === "y");
+		expect(y?.line).toBe(2);
+	});
+
+	test("tracks column numbers", () => {
+		const tokens = new Lexer("new x = 10;").tokenize();
+		expect(tokens[0]?.column).toBe(1); // new
+		expect(tokens[1]?.column).toBe(5); // x
+		expect(tokens[2]?.column).toBe(7); // =
+		expect(tokens[3]?.column).toBe(9); // 10
+	});
+
+	test("handles identifiers with underscores", () => {
+		const tokens = new Lexer("my_var _private").tokenize();
+		expect(tokens[0]).toMatchObject({
+			type: TokenType.Identifier,
+			value: "my_var",
+		});
+		expect(tokens[1]).toMatchObject({
+			type: TokenType.Identifier,
+			value: "_private",
+		});
+	});
+
+	test("handles identifiers with numbers", () => {
+		const tokens = new Lexer("x1 var2name").tokenize();
+		expect(tokens[0]).toMatchObject({
+			type: TokenType.Identifier,
+			value: "x1",
+		});
+		expect(tokens[1]).toMatchObject({
+			type: TokenType.Identifier,
+			value: "var2name",
+		});
+	});
+
+	test("throws on unexpected character", () => {
+		expect(() => new Lexer("@").tokenize()).toThrow("Unexpected character '@'");
+	});
+
+	test("handles empty input", () => {
+		const tokens = new Lexer("").tokenize();
+		expect(tokens).toHaveLength(1);
+		expect(tokens[0]).toMatchObject({ type: TokenType.EOF });
+	});
+
+	test("handles whitespace only", () => {
+		const tokens = new Lexer("   \t\n  \n").tokenize();
+		expect(tokens).toHaveLength(1);
+		expect(tokens[0]).toMatchObject({ type: TokenType.EOF });
+	});
+});
diff --git a/tests/parser.test.ts b/tests/parser.test.ts
new file mode 100644
index 0000000..6e6c6c3
--- /dev/null
+++ b/tests/parser.test.ts
@@ -0,0 +1,133 @@
+import { describe, expect, test } from "bun:test";
+import { Lexer } from "../src/lexer";
+import type { Program } from "../src/parser";
+import { Parser } from "../src/parser";
+
+function parse(source: string): Program {
+	const tokens = new Lexer(source).tokenize();
+	return new Parser(tokens).parse();
+}
+
+describe("Parser", () => {
+	test("parses variable declaration", () => {
+		const ast = parse("new x = 10;");
+		expect(ast.body).toHaveLength(1);
+		expect(ast.body[0]).toMatchObject({
+			type: "VariableDeclaration",
+			name: "x",
+			value: { type: "NumberLiteral", value: 10 },
+		});
+	});
+
+	test("parses assignment", () => {
+		const ast = parse("new x = 1;\nx = 5;");
+		expect(ast.body[1]).toMatchObject({
+			type: "Assignment",
+			name: "x",
+			value: { type: "NumberLiteral", value: 5 },
+		});
+	});
+
+	test("parses print statement", () => {
+		const ast = parse("print(42);");
+		expect(ast.body[0]).toMatchObject({
+			type: "PrintStatement",
+			value: { type: "NumberLiteral", value: 42 },
+		});
+	});
+
+	test("parses binary expression with addition", () => {
+		const ast = parse("new x = 1 + 2;");
+		expect(ast.body[0]).toMatchObject({
+			type: "VariableDeclaration",
+			name: "x",
+			value: {
+				type: "BinaryExpression",
+				operator: "+",
+				left: { type: "NumberLiteral", value: 1 },
+				right: { type: "NumberLiteral", value: 2 },
+			},
+		});
+	});
+
+	test("parses binary expression with subtraction", () => {
+		const ast = parse("new x = 10 - 3;");
+		expect(ast.body[0]).toMatchObject({
+			type: "VariableDeclaration",
+			name: "x",
+			value: {
+				type: "BinaryExpression",
+				operator: "-",
+				left: { type: "NumberLiteral", value: 10 },
+				right: { type: "NumberLiteral", value: 3 },
+			},
+		});
+	});
+
+	test("parses chained binary expressions left-to-right", () => {
+		const ast = parse("new x = 1 + 2 - 3;");
+		// Should be ((1 + 2) - 3)
+		expect(ast.body[0]).toMatchObject({
+			type: "VariableDeclaration",
+			name: "x",
+			value: {
+				type: "BinaryExpression",
+				operator: "-",
+				left: {
+					type: "BinaryExpression",
+					operator: "+",
+					left: { type: "NumberLiteral", value: 1 },
+					right: { type: "NumberLiteral", value: 2 },
+				},
+				right: { type: "NumberLiteral", value: 3 },
+			},
+		});
+	});
+
+	test("parses identifier in expression", () => {
+		const ast = parse("new x = 1;\nnew y = x + 2;");
+		expect(ast.body[1]).toMatchObject({
+			type: "VariableDeclaration",
+			name: "y",
+			value: {
+				type: "BinaryExpression",
+				operator: "+",
+				left: { type: "Identifier", name: "x" },
+				right: { type: "NumberLiteral", value: 2 },
+			},
+		});
+	});
+
+	test("parses print with expression", () => {
+		const ast = parse("print(1 + 2);");
+		expect(ast.body[0]).toMatchObject({
+			type: "PrintStatement",
+			value: {
+				type: "BinaryExpression",
+				operator: "+",
+				left: { type: "NumberLiteral", value: 1 },
+				right: { type: "NumberLiteral", value: 2 },
+			},
+		});
+	});
+
+	test("parses multiple statements", () => {
+		const ast = parse("new a = 1;\nnew b = 2;\nprint(a + b);");
+		expect(ast.body).toHaveLength(3);
+		expect(ast.body[0]?.type).toBe("VariableDeclaration");
+		expect(ast.body[1]?.type).toBe("VariableDeclaration");
+		expect(ast.body[2]?.type).toBe("PrintStatement");
+	});
+
+	test("throws on missing semicolon", () => {
+		expect(() => parse("new x = 10")).toThrow("Expected Semicolon");
+	});
+
+	test("throws on missing equals in declaration", () => {
+		expect(() => parse("new x 10;")).toThrow("Expected Equals");
+	});
+
+	test("throws on unexpected token", () => {
+		expect(() => parse("= 10;")).toThrow("Unexpected token");
+	});
+});