This document provides an overview of microservice security using Spring Security 5.1, OAuth 2.0, and OpenID Connect. It defines OAuth 2.0 and its authorization framework, describing how applications can be authorized to access user data. It outlines the authorization code grant flow and other grant types, including how applications register and use client IDs and secrets. JSON Web Tokens are discussed as an access token format. OpenID Connect is described as extending OAuth 2.0 to provide authentication via ID tokens. Key components like access tokens, refresh tokens, and the client credentials flow are also summarized.