Smart environments that respond to humans are expected to become a part of our everyday life [1,32,44]. Low cost and low power sensors that enable detecting humans and their movements are seen as the major trend in the technology development that is needed to enable the future human-responsive environments. This chapter presents an overview and analysis of the ”imaging” sensors that can be used for sensing movement and presence of people. Main sensing principles are described for the major available technologies. The ”video” data obtained with each sensor will be very different. The main problems and the typical algorithms for detecting movement and people are presented for each sensor. Finally, the costs, the development trends and other practical aspects are highlighted and the sensors are compared.