Acoustic pump

An acoustically driven pump comprises a chamber 20 having opposed first and second side walls 11, 12, and first and second end walls 13, 14. At least a portion of the first side wall is defined by means for propagating an acoustic wave (e.g. a piezoelectric actuator) within said chamber at a predetermined wavelength in order to generate a standing wave between the first and second side walls. One or more apertures 15, 16 are provided in each of the first and second end walls such that pressure differentials within the chamber adjacent to the aperture(s) caused by the standing wave cause fluid to flow through the aperture(s) in a direction substantially parallel to the first side wall. One or more of the apertures may be tapered, and/or provided with a backflow lip, to provide greater flow resistance in one direction than the other. The chamber may be provided in a further housing, with a region being defined there between.