This paper introduces a unifying reference framework for classifying and developing user interfaces that support multiple contexts of use, or multi-target user interfaces. The framework decomposes a context of use into three facets: users, hardware and software platforms, and the physical environment. It defines context-sensitive user interfaces as those capable of being aware of and reacting to changes in the context. The framework structures the development life cycle into four levels of abstraction: task and concepts, abstract user interface, concrete user interface, and final user interface. These levels are connected by relationships of reification and abstraction, facilitating understanding and comparison of various methods and tools. The framework also introduces the concept of plastic user interfaces, which adapt to context changes while preserving usability. The paper discusses the framework's application in multi-targeting and plasticity, and provides examples of how it can be used to design and develop context-sensitive user interfaces.This paper introduces a unifying reference framework for classifying and developing user interfaces that support multiple contexts of use, or multi-target user interfaces. The framework decomposes a context of use into three facets: users, hardware and software platforms, and the physical environment. It defines context-sensitive user interfaces as those capable of being aware of and reacting to changes in the context. The framework structures the development life cycle into four levels of abstraction: task and concepts, abstract user interface, concrete user interface, and final user interface. These levels are connected by relationships of reification and abstraction, facilitating understanding and comparison of various methods and tools. The framework also introduces the concept of plastic user interfaces, which adapt to context changes while preserving usability. The paper discusses the framework's application in multi-targeting and plasticity, and provides examples of how it can be used to design and develop context-sensitive user interfaces.