Vision systems are becoming increasingly important sources of information for automated inspection and assembly tasks. Spatial information along directions transverse to the viewing axis is easily obtainable with imaging systems and one-or two-dimensional detectors. A challenge for optics is to develop fast, accurate range sensors. This paper reviews range sensing techniques. Four basic categories of range sensing techniques are discussed: geometric techniques, time-of-flight techniques, interferometric techniques and diffraction techniques. The basic principles are elucidated and general comparisons are made between the groups. Representative examples are given of many different approaches.