Single sign-on (SSO) is an authentication mechanism that allows users to access multiple applications with one set of credentials, enhancing user experience and security by reducing the need for multiple passwords. This guide, written by Gideon Idoko, navigates through the basics of SSO, its benefits, drawbacks, and different types, as well as implementation methods. It provides insights into implementing SSO from scratch using protocols like SAML and OIDC, detailing both an in-house approach and using third-party solutions like Descope. Implementing SSO from scratch involves technical challenges and potential security risks, such as lack of token encryption and vulnerability to CSRF, whereas Descope offers a streamlined, secure, and compliant solution with prebuilt features and scalability for enterprise needs. The guide includes practical steps for setting up SSO with Okta as the Identity Provider and implementing it in a Next.js application, emphasizing the trade-offs between custom-built and third-party solutions.