This document reviews the OpenID authentication framework. It begins with an introduction explaining the problem of users needing multiple usernames and passwords for different websites. It then provides an overview of OpenID as a decentralized single sign-on protocol that allows users to access websites using a single username and password. The document outlines the OpenID authentication process involving an OpenID provider, relying party, and user. It concludes by discussing how OpenID has been adopted by many large websites and integrated with other identity frameworks to improve security.