Applications and libraries with a pluggable architecture allow developers to add custom functionality. Plugins can customize user interfaces, create new workflows, and interface with legacy systems. Designing a plugin is often difficult - documentation is sparse, outdated, or non-existent. You end up diving into a unfamiliar codebase to figure out what to do. This talk examines Pluggable Architecture by creating a custom plugin system: we will design an interface, think about where to hook in custom behavior, and discuss testing techniques. Understanding these principles will enable us to write custom plugins for third-party libraries. Extend the functionality of your favourite library without touching existing code!