A pump is formed by a housing (10) having a fluid inlet (11) and a fluid outlet (12) and containing a rotor (15) forming with the housing (10) chambers (17a, 17b) that, on rotation of the rotor (15) by a drive, convey fluid from the inlet (11) to the outlet (12) to pump the fluid. A seal assembly (14) is arranged between the outlet (12) and the inlet (11). The seal assembly (14) includes a membrane (21) that contacts the rotor (15) and a flexible resilient spring member (22, 28, 35, 37, 40) that provides a force urging the membrane (21) against the rotor (15). The spring member (22, 28, 35, 37, 40) thus, on rotation of the rotor (15), moves radially relative to the axis of rotation of the rotor (15) and is arranged to provide a force on the rotor (15) via the membrane (21) that is constant and a minimum to maintain a seal between the rotor (15) and the seal (14) for a given outlet pressure of the pumped fluid.