diff --git a/.vscode/launch.json b/.vscode/launch.json
index 80f02b8..8de82fc 100644
--- a/.vscode/launch.json
+++ b/.vscode/launch.json
@@ -27,6 +27,17 @@
                 "${workspaceFolder}/out/**/*.js",
                 "${workspaceFolder}/node_modules/langium/**/*.js"
             ]
+        },
+        {
+            "type": "node",
+            "request": "launch",
+            "name": "Debug Current Test File",
+            "autoAttachChildProcesses": true,
+            "skipFiles": ["<node_internals>/**", "**/node_modules/**"],
+            "program": "${workspaceRoot}/node_modules/vitest/vitest.mjs",
+            "args": ["run", "${relativeFile}"],
+            "smartStep": true,
+            "console": "integratedTerminal"           
         }
     ]
 }
diff --git a/package.json b/package.json
index e133faa..d1755ea 100644
--- a/package.json
+++ b/package.json
@@ -48,7 +48,8 @@
         "vscode:prepublish": "npm run build && npm run lint",
         "build": "tsc -b tsconfig.json",
         "watch": "tsc -b tsconfig.json --watch",
-        "lint": "eslint src --ext ts",
+        "test": "vitest run",
+        "lint": "eslint src test --ext ts",
         "langium:generate": "langium generate",
         "langium:watch": "langium generate --watch"
     },
@@ -70,6 +71,7 @@
         "@typescript-eslint/parser": "^4.14.1",
         "eslint": "^7.19.0",
         "langium-cli": "1.0.0",
-        "typescript": "^4.6.2"
+        "typescript": "^4.6.2",
+        "vitest": "^0.34.2"
     }
 }
diff --git a/src/interpreter/runner.ts b/src/interpreter/runner.ts
index 76a8b2e..f69e558 100644
--- a/src/interpreter/runner.ts
+++ b/src/interpreter/runner.ts
@@ -3,7 +3,7 @@ import { BinaryExpression, Expression, isBinaryExpression, isBooleanExpression,
 import { createLoxServices } from "../language-server/lox-module";
 import { v4 } from 'uuid';
 import { URI } from "vscode-uri";
-import { CancellationToken } from "vscode-languageclient";
+import { CancellationToken } from 'vscode-jsonrpc';
 
 export interface InterpreterContext {
     log: (value: unknown) => MaybePromise<void>
@@ -261,10 +261,10 @@ async function runMemberCall(memberCall: MemberCall, context: RunnerContext): Pr
     }
 
     if (memberCall.explicitOperationCall) {
-        if (isFunctionDeclaration(ref)) {
+        if (isFunctionDeclaration(value)) {
             const args = await Promise.all(memberCall.arguments.map(e => runExpression(e, context)));
             context.variables.enter();
-            const names = ref.parameters.map(e => e.name);
+            const names = value.parameters.map(e => e.name);
             for (let i = 0; i < args.length; i++) {
                 context.variables.push(names[i], args[i]);
             }
@@ -272,7 +272,7 @@ async function runMemberCall(memberCall: MemberCall, context: RunnerContext): Pr
             const returnFn: ReturnFunction = (returnValue) => {
                 functionValue = returnValue;
             }
-            await runLoxElement(ref.body, context, returnFn);
+            await runLoxElement(value.body, context, returnFn);
             context.variables.leave();
             return functionValue;
         } else {
diff --git a/test/function.test.ts b/test/function.test.ts
new file mode 100644
index 0000000..44d7d31
--- /dev/null
+++ b/test/function.test.ts
@@ -0,0 +1,118 @@
+import { runInterpreter } from '../src/interpreter/runner.js';
+import { expect, test } from 'vitest';
+
+test('identity function', async() => {
+  const input = `
+    fun returnSum(a: number, b: number): number {
+      print "returnSum called";
+      return a + b;
+    }
+
+    fun identity(a: (number, number) => number): (number, number) => number {
+      print "identity called";
+      return a;
+    }
+
+    print identity(returnSum)(27, 15); // prints "42";
+  `;
+
+  const expectedOutput = `
+    identity called
+    returnSum called
+    42
+  `;
+
+  await runInterpreterAndAssertOutput(input, expectedOutput);
+});
+
+test('pass reference to function and call it', async() => {
+  const input = `
+    fun aFunction(aLambda: (number) => number, aNumber: number): number {
+        print "aFunction called";
+        return aLambda(aNumber);
+    }
+
+    fun aTimesTwo(a: number): number {
+        print "aTimeTwo called";
+        return a * 2;
+    }
+
+    var result = aFunction(aTimesTwo, 9);
+    print result;
+  `;
+
+  const expectedOutput = `
+    aFunction called
+    aTimeTwo called
+    18
+  `;
+
+  await runInterpreterAndAssertOutput(input, expectedOutput);
+});
+
+test('Closure 1', async() => {
+  const input = `
+    // So far fails with: No variable 'outside' defined
+
+    fun returnFunction(): () => void {
+        var outside = "outside";
+    
+        fun inner(): void {
+            print outside;
+        }
+    
+        return inner;
+    }
+    
+    var fn = returnFunction();
+    fn();
+  `;
+
+  const expectedOutput = `
+  `;
+
+  await runInterpreterAndAssertOutput(input, expectedOutput);
+});
+
+test('Closure 2', async() => {
+  const input = `
+    // So far fails with: No variable 'exponent' defined
+
+    fun power(exponent: number): (number) => number {
+      fun applyPower(base: number): number {
+          var current = 1;
+            for (var i = 0; i < exponent; i = i + 1) {
+              current = current * base;
+            }
+            return current;
+        }
+        return applyPower;
+      }
+    
+    var cube = power(3);
+    
+    print cube(1);
+    print cube(2);
+    print cube(3);
+    print cube(4);
+    print cube(5);
+  `;
+
+  const expectedOutput = `
+  `;
+
+  await runInterpreterAndAssertOutput(input, expectedOutput);
+});
+
+
+async function runInterpreterAndAssertOutput(input: string, expectedOutput: string) {
+  // TODO call valication before ?!?
+  let output = "";
+  await runInterpreter(input, {
+    log: value => {
+      output = output.concat(`${value}`);
+    }
+  });
+  expect(output.replace(/\s/g, "")).toBe(expectedOutput.replace(/\s/g, ""));
+}
+
diff --git a/test/grammar-parse.test.ts b/test/grammar-parse.test.ts
new file mode 100644
index 0000000..b49e1cd
--- /dev/null
+++ b/test/grammar-parse.test.ts
@@ -0,0 +1,28 @@
+import type { LoxProgram } from '../src/language-server/generated/ast.js';
+import { createLoxServices } from '../src/language-server/lox-module.js';
+import { EmptyFileSystem } from 'langium';
+import { parseHelper } from 'langium/test';
+import { test } from 'vitest';
+
+
+test('parse', async() => {
+  const services = createLoxServices(EmptyFileSystem).Lox;
+  const parse = parseHelper<LoxProgram>(services);
+
+  const input = `
+  fun returnSum(a: number, b: number): number {
+      return a + b;
+  }
+
+  // Closures
+
+  fun identity(a: (number, number) => number): (number, number) => number {
+      return a;
+  }
+
+  print identity(returnSum)(1, 2); // prints "3";
+  `
+
+  const ast = await parse(input);
+  ast.parseResult;
+});
diff --git a/tsconfig.json b/tsconfig.json
index ca965e3..3d771e6 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -14,7 +14,8 @@
     "forceConsistentCasingInFileNames": true
   },
   "include": [
-     "src/**/*.ts"
+     "src/**/*",
+     "test/**/*"
   ],
   "exclude": [
      "out",
diff --git a/tsconfig.src.json b/tsconfig.src.json
new file mode 100644
index 0000000..fa0c6ea
--- /dev/null
+++ b/tsconfig.src.json
@@ -0,0 +1,10 @@
+{
+    "extends": "./tsconfig.json",
+    "compilerOptions": {
+      "rootDir": "src",
+      "outDir": "out"
+    },
+    "include": [
+      "src/**/*"
+    ]
+}
\ No newline at end of file
diff --git a/tsconfig.test.json b/tsconfig.test.json
new file mode 100644
index 0000000..13d368d
--- /dev/null
+++ b/tsconfig.test.json
@@ -0,0 +1,13 @@
+{
+    "extends": "./tsconfig.src.json",
+    "compilerOptions": {
+      "noEmit": true,
+      "rootDir": "test"
+    },
+    "references": [{
+      "path": "./tsconfig.src.json"
+    }],
+    "include": [
+      "test/**/*",
+    ]
+  }
\ No newline at end of file
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..015d143
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,16 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+    test: {
+        coverage: {
+            provider: 'v8',
+            reporter: ['text', 'html'],
+            include: ['src'],
+            exclude: ['**/generated'],
+        },
+        deps: {
+            interopDefault: true
+        },
+        include: ['test/*.test.ts']
+    }
+})
\ No newline at end of file