A miniature non-return valve comprises a circular recess (7) with an inlet (3) at its centre, an annular groove (17) coaxial with the recess and communicating with the recess at a number of points (9) within the groove, and an outlet duct (19) communicating with the groove. Fluid entering the inlet passes through the recess, the annular groove and the outlet duct substantially unimpeded, whereas fluid entering the outlet duct forms a vortex in the recess so that flow of that fluid to the inlet is inhibited. Control fluid may be fed into the recess to initiate or enhance formation of the vortex. The inlet and the circular recess may be provided in first and second substrates (1,5) respectively, and the annular groove and the outlet duct may be provided in a third substrate (15),all by a micromachining process, the substrates being bonded together in a stack. The substrates may be formed of silicon.