The computational model associated with ubiquitous computing is quite different to traditional models. Large numbers of sensors are embedded into the physical environment, providing varied data and services to users and applications. In such environments the task of building context-aware applications, which use information about their environment in order to adapt, is a complex one. Contributing factors include variable availability of resources, the need to aggregate information from multiple sources to derive high-level context, and the ongoing evolution of technologies deployed in the environment.