Kustomize is a popular tool for declarative, template-free management of Kubernetes configuration. Under the hood, it works by defining a graph of Kubernetes resources and applying a pipeline of declaratively specified transformations to its nodes. A lesser known fact is that users aren't limited to the transformations that Kustomize comes with out of the box: it's extensible! The way Kustomize extensions work has evolved over time towards a model that closely resembles Custom Resources, but on the client side! In this talk, Katrina and Jeff will briefly review the history and future of Kustomize extensions, and then dive into practical applications. The talk will showcase how these CR-style extensions can address common yet idiosyncratic application needs, as well as how platform builders can leverage them to manage declarative configuration at scale. Attendees will leave with practical knowledge of what makes a good extension and how to use Kustomize’s kyaml tools to start building.