The increasing proliferation of computational devices has introduced the need for applications to run on multiple platforms in different physical environments. Providing a user interface specially crafted for each context of use is extremely costly and may result in inconsistent behavior. User interfaces must now be capable of adapting to multiple sources of variation. This paper presents a unifying framework that structures the development process of plastic user interfaces. A plastic user interface is capable of adapting to variations of the context of use while preserving usability. The reference framework has guided the design of ARTStudio, a model-based tool that supports the plastic development of user interfaces. The framework as well as ARTStudio are illustrated with a common running example: a home heating control system.