Methods and apparatus, including computer program products, providing and using composite extension definitions. A composite extension definition has one or more extension definitions, each of which can be either an atomic or a non-atomic extension definition. In one aspect, a program has instructions to create a composite extension definition, to generate composite metadata that represents the composite extension definition, and to store the composite metadata for later use. The program can include instructions to receive input specifying dependency relationships between or among extensions defined by the extension definitions and to include the relationships in the composite metadata. Composite extension definitions can be created by an application developer for use by customers to guide them in implementing and deploying extensions to a computer program application.