A valve of the invention includes a valve body having an inlet, an outlet, and a flow passage connecting the inlet and outlet, the inlet, the outlet and the flow passage being in general axial alignment within the valve body. A valve seat surrounds the flow passage and a valve closure member is movable towards and away from the valve seat to close and open the valve respectively. The valve closure member is a poppet-type closure member axially mounted to a stem. The stem has a plurality of pistons spaced apart along the length thereof, and the valve body has a plurality of cylinders fixed relative thereto, the pistons each being slidable in a respective cylinder. The valve closure member is movable by varying the pressure in the cylinders. Preferably each of the cylinders has an outlet port through a wall thereof which is exposed when the closure member has moved towards or into a fully open position, pressure within said cylinders dissipating through those outlet ports when the outlet ports are exposed.