diff --git a/.gitignore b/.gitignore
index b512c09..0c4f1f6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,3 @@
-node_modules
\ No newline at end of file
+node_modules
+test/.env.example
+env.d.ts
\ No newline at end of file
diff --git a/README.md b/README.md
index 6e73ab7..5898b5d 100644
--- a/README.md
+++ b/README.md
@@ -61,6 +61,8 @@ npx gen-env-types path/to/.env
   -h, --help                  Show usage information
   -o, --types-output          Output name/path for types file | defaults to `env.d.ts`
   -e, --example-env-path      Path to save .env.example file
+  -O,  --optional [vars]      Make some of the environment variables optional.
+                              Accepts a list of environment variables to be made optional.
   -r, --rename-example-env    Custom name for .env example output file | defaults to `env.example` if omitted
   -k, --keep-comments         Keep comments/blank lines in .env example output file | defaults to false if omitted.
                               Not accepting the value. When specified, it will be true.
diff --git a/gen-env-types.js b/gen-env-types.js
index adb3213..f1de084 100644
--- a/gen-env-types.js
+++ b/gen-env-types.js
@@ -26,6 +26,8 @@ const printHelp = (exitCode) => {
   -h,  --help                  Show usage information
   -o,  --types-output          Output name/path for types file | defaults to \`env.d.ts\`
   -e,  --example-env-path      Path to save .env.example file
+  -O,  --optional [vars]       Make some of the environment variables optional.
+                               Accepts a list of environment variables to be made optional.
   -r,  --rename-example-env    Custom name for .env example output file | defaults to \`env.example\` if omitted
   -k,  --keep-comments         Keep comments/blank lines in .env example output file | defaults to false if omitted.
                                Not accepting the value. When specified, it will be true.
@@ -45,6 +47,7 @@ const parseArgs = (args) => {
     typesOutput: "env.d.ts",
     exampleEnvOutput: ".env.example",
     keepComments: false,
+    listOfOptionalVariables: []
   };
 
   while (args.length > 0) {
@@ -61,6 +64,16 @@ const parseArgs = (args) => {
       case "--version":
         cliConfig.version = true;
         break;
+      case "-O":
+      case "--optional":
+        const listOfOptionalVariables = args.shift();
+        if (!listOfOptionalVariables) {
+          showError(
+            "Expected a list of optional variables, bad input: " + listOfOptionalVariables
+          );
+        }
+        cliConfig.listOfOptionalVariables = listOfOptionalVariables;
+        break;
       case "-o":
       case "--types-output":
         const typesOutput = args.shift();
@@ -117,10 +130,10 @@ if (!cliConfig.envPath) {
   printHelp(1);
 }
 if (cliConfig.help) {
-  return printHelp(0);
+  printHelp(0);
 }
 if (cliConfig.version) {
-  return printVersion();
+  printVersion();
 }
 
 const envString = readFileSync(cliConfig.envPath, {
@@ -141,16 +154,17 @@ function writeEnvTypes(path) {
     interface ProcessEnv {
       ${filteredEnvString
     .map(({key}, i) => {
+      const isKeyOptional = cliConfig.listOfOptionalVariables.length > 0 && cliConfig.listOfOptionalVariables.includes(key);
       if (!existingModuleDeclaration) {
-        return `${i ? "      " : ""}${key}: string;`;
+        return `${i ? "      " : ""}${key}${isKeyOptional ? '?' : ''}: string;`;
       }
 
       const existingPropertySignature = existingModuleDeclaration
         .split("\n")
-        .find((line) => line.includes(`${key}:`));
+        .find((line) => line.includes(`${key}:`) || line.includes(`${key}?:`));
 
       if (!existingPropertySignature) {
-        return `${i ? "      " : ""}${key}: string;`;
+        return `${i ? "      " : ""}${key}${isKeyOptional ? '?' : ''}: string;`;
       }
 
       return `${i ? "      " : ""}${existingPropertySignature.trim()}`;
@@ -166,10 +180,12 @@ export {}
   writeFileSync(path, moduleDeclaration);
 
   console.log("Wrote env types to: ", path);
+
+  return moduleDeclaration;
 }
 
 function writeExampleEnv(parsedExistingEnvString, path, isNew) {
-  const out = (cliConfig.keepComments? parsedEnvString: filteredEnvString)
+  const out = (cliConfig.keepComments ? parsedEnvString: filteredEnvString)
     .map(({key, isEnvVar,value}) => {
       if(isEnvVar) return `${key}=`;
       // Comment or blank value
@@ -199,8 +215,17 @@ if (cliConfig.exampleEnvPath) {
       readFileSync(outputExampleEnvPath, { encoding: "utf-8" })
     );
 
-    return writeExampleEnv(parsedExistingEnvString, outputExampleEnvPath);
+    writeExampleEnv(parsedExistingEnvString, outputExampleEnvPath);
   }
 
   writeExampleEnv(filteredEnvString, outputExampleEnvPath, true);
 }
+
+module.exports = {
+    /**
+   * @description Writes environment types to a file
+   * @param {string} path
+   * @returns the content of the created file
+   */
+  writeEnvTypes
+}
\ No newline at end of file
diff --git a/gen-env-types.test.js b/gen-env-types.test.js
new file mode 100644
index 0000000..1b196c0
--- /dev/null
+++ b/gen-env-types.test.js
@@ -0,0 +1,20 @@
+const { writeEnvTypes } = require('./gen-env-types');
+
+describe('Environment variable types generator', function () {
+    it('should accept a list of optional variables', function () {        
+        const result = writeEnvTypes('env.d.ts');
+        expect(result).toMatchInlineSnapshot(`
+"declare global {
+  namespace NodeJS {
+    interface ProcessEnv {
+      OPTIONAL_SECRET?: string;
+      REQUIRED_SECRET: string;
+    }
+  }
+}
+
+export {}
+"
+`);
+      });
+});
\ No newline at end of file
diff --git a/jest.config.js b/jest.config.js
new file mode 100644
index 0000000..2fe4535
--- /dev/null
+++ b/jest.config.js
@@ -0,0 +1,3 @@
+module.exports = {
+    setupFiles: ['./testSetup'],
+};
\ No newline at end of file
diff --git a/test/.env.test b/test/.env.test
new file mode 100644
index 0000000..2f0fa35
--- /dev/null
+++ b/test/.env.test
@@ -0,0 +1,2 @@
+OPTIONAL_SECRET=test_optional
+REQUIRED_SECRET=test_required
\ No newline at end of file
diff --git a/testSetup.js b/testSetup.js
new file mode 100644
index 0000000..20febb1
--- /dev/null
+++ b/testSetup.js
@@ -0,0 +1 @@
+process.argv = ['node', 'jest', 'test/.env.test', '--types-output', 'env.d.ts', '--example-env-path', 'test', '--optional', '[OPTIONAL_SECRET]'];
\ No newline at end of file