Two-factor authentication is implemented using the Vonage Verify API and Express.js in this tutorial. The application has three pages: one to ask for a mobile number, another to provide the verification code sent via SMS or phone call, and a final page to display if the verification was successful. To complete this tutorial, you need Node.js installed on your machine, a Vonage API account with an API key and secret, and a virtual phone number. The application uses Express.js as the web framework, body-parser for parsing requests, Nunjucks for templating, and Nexmo for sending SMS or making calls to deliver verification codes. The finished code example is available at https://github.com/nexmo-community/verify-node-express.