Abstract: The invention provides a valve assembly (10) including a valve body (12) defining a passage (14) having an inlet (16) and an outlet (18), the body having a valve seat (20); and a valve member for opening and closing the passage, the valve member being movable relative to the body between an open position, and a closed position wherein the valve member seals against the valve seat. The valve assembly also includes an actuator assembly (28) for moving the valve member between the open and closed positions, the actuator assembly including a shaft (30) movable relative to the body and connected to the valve member, and two primary pressure chambers (32, 34). The primary chambers are divided into a relatively high-pressure chamber (32.2, 34.2) and a relatively low-pressure chamber (32.1, 34.1) by movable sealing means connected to the shaft. The arrangement is such that pressurisation of the relatively high-pressure chambers moves the valve member towards the closed position.