The imminent mass deployment of pervasive computing technologies such as WSN and RFID tags, together with the increasing participation of the Web community, will soon make available an incredible amount of information about the physical and social worlds and their processes. This opens up the possibility of exploiting all such information for the provisioning of context- aware services facilitating users in gathering information about the world, interacting with it, and understanding it. Specifically, in this book, I propose a simple model for representing "facts" about the physical world in a structured fashion, besides the design and implementation of a general infrastructure for "context-aware" services able to organize and provide access to contextual data. Once the above elements are well integrated in a framework (or paradigm) for application development, my thesis demonstrates how users provided of a computing device (as a modern mobile phone or PDA) and of technologies enabling the interaction with the surrounding world (as RFID reader, GPS antenna or a Wireless Sensor Network) gain access to a number of futuristic and unexplored services and applications.