A building comprising a plurality of rooms (10) includes a room occupancy sensing apparatus. A light source (20) emits a series of light pulses (22), a plurality of waveguides deliver light from the light source to output nodes (60) located in the rooms, and a signal capture unit (30) receives output signals resulting from light reflected by objects in the rooms. The apparatus detects movement, of for example a person (40), in a room and ascertains the room concerned by virtue of (i) detecting a difference between the shape of the waveform of the signal (24 i) received at the signal capture unit (30) in response to a first emitted light pulse and the shape of the waveform of the signal (24m) received at the signal capture unit in response to a second emitted light pulse and (ii) relating said reflected light pulses to the appropriate output node and therefore to the room (10) associated with that output node (60).