This book describes recent work on active sensors for mobile robots. An active sensor interacts with its surroundings to supply data on demand for a particular function, gathering and abstracting information according to need rather than acting as a generic data gatherer. Details of the physical operation are hidden.The book deals mainly with active range sensors, which provide rapid information for local planning, describing extraction of two-dimensional features such as lines, corners and cylinders to reconstruct a plan of a building. It is structured according to the physical principles of the sensors, since to a large extent these determine the function of the sensors and the methods of processing. Recent work using sonar, optoelectronic sensors and radar is described. Sections on vision and on sensor management develop the idea of software adaptation for efficient operation in a changing environment.