Finding the position of a target based on measurements from an array of spatially separated sensors has been an important problem in radar, sonar, global positioning system, mobile communications, multimedia and wireless sensor networks. Time-of-arrival (TOA), time-difference-of-arrival (TDOA), received signal strength (RSS) and direction-of-arrival (DOA) of the emitted signal are commonly used measurements for source localization. Basically, TOAs, TDOAs and RSSs provide the distance information between the source and sensors while DOAs are the source bearings relative to the receivers. However, locating the target is not a trivial task because these measurements have nonlinear relationships with the source position. In this presentation, representative applications of source localization are first described. Nonlinear and linear solutions for TOA-, TDOA-, RSS- and DOA-based positioning are then presented.