The future development of air surveillance sensors is described by applying basic principles that do not require detailed technical analysis. The main purpose is to present a simple description of how future sensor systems can be designed to detect new air targets. For this purpose sensors are considered from four different aspects: range, weather dependence, information content and ray paths, which is sufficient to give a complete picture of how a future system should be constructed. The analysis demonstrates that radar is unique as a sensor of airborne targets. No other sensors can replace radar in the foreseeable future and it is consequently necessary to investigate how well future radar systems can detect new threats like Unmanned aerial vehicles (UAVs) and cruise missiles. Radar has reached a high level of technical maturity and in practice the main restriction is often the curvature of earth. The horizon will limit radar range, especially for targets flying at a low altitude, and although certain radars can see beyond the horizon Over-The-Horizon Radar (OTH-radar), their wavelengths are so long that it is difficult to detect small targets. Low flying targets require radars to be located in high masts or on airborne platforms. The alternative is to collect data from many short range sensors, but this removes the main advantage of radar, its long range. In order to describe how air surveillance and the air force should be organized in the future the threat presented by missiles and UAVs must be analysed in depth. The methods presented here allows one to perform an elementary analysis before a detailed technical and economical analysis is attempted.