In a wireless sensor network having a plurality of mobile sensor nodes (62, 63, 64, 65), a sink (70) is provided and re-positioned as required by assigning the role of sink to one of the mobile sensors at a time. Re-positioning involves finding candidate sinks in the vicinity of a current sink, a temporary sink or a central candidate in the network; judging the suitability of each candidate, for example by estimating the data throughput likely to be achieved if each candidate is selected as the new sink; and transferring the role of sink to the best candidate found. The temporary sink or central candidate may be selected with the aid of a positioning system (80) of the network. Suitability of a candidate sink may take into account a desired transmission rate of each sensor node in the vicinity of a candidate, this desired rate being found from a 3-D look-up table of each sensor.