This article considers the importance of context in mobile systems. It considers a range of context-related issues and focus on location as a key issue for mobile systems. A design framework is described consisting of taxonomies of location, mobility, population, and device awareness. The design framework informs the construction of a semantic model of space for mobile systems. The semantic model is reflected in a computational model built on a distributed platform that allows contextual information to be shared across a number of mobile devices. The framework supports the design of interactive mobile systems while the platform supports their rapid development.