A radio-location method of locating a target device that is contained within a region of space, comprising obtaining a first set of detected signals by detecting, at a plurality of points in space, a beacon signal from the target device, or by detecting at the target device, beacon signals sent from one or more radio transmitters in the region of space; determining a current estimate of the target device's location; obtaining successive sets of detected signals, for each successive set of detected signals: determining one or more candidate locations for the target device; comparing the candidate locations with a radio map of the region of space, identifying one or more feasible candidate locations that are consistent with the radio map; determining a new estimate of the target device's location based on the one or more feasible candidate locations; and setting the new estimate as the current estimate of the target device's location.