This document discusses Open Policy Agent (OPA), an open source general-purpose policy engine. It provides examples of how OPA can be used to enforce various types of policies across complex environments in a flexible way. OPA treats policy decisions as separate from enforcement, stores policies and data in-memory, and uses partial evaluation and indexing to evaluate policies efficiently. It allows policies to be written declaratively using Rego and enforced for services, infrastructure, and other resources regardless of how they are implemented. The document demonstrates examples of using OPA for authorization, RBAC, and other policies across multiple domains.