An actuator assembly for use with a shut-off valve (100; 400; 604; 700; 800; 900), the valve comprising a valve body (104; 404), a bonnet (108; 408) coupled to the valve body, and a movable stem (112; 410) supported by the bonnet for controlling fluid flow through the valve body, wherein the actuator assembly comprises: a mounting assembly comprising a mounting bracket configured to be coupled to the bonnet of the shut-off valve and a mounting sleeve configured to be coupled to the movable stem of the shut-off valve, the mounting sleeve being movable relative to the mounting bracket, the mounting assembly further comprising a first arm coupled to the mounting bracket and a second arm coupled to and extending outward from the mounting sleeve; and an actuator configured to be operatively coupled to the shut-off valve via the mounting assembly, the actuator comprising: a body defined by a first body portion and a second body portion movable relative to the first body portion to expand or contract the body; a first end coupled to the first body portion and comprising a first opening arranged to receive the first arm of the mounting assembly; a second end coupled to the second body portion; and an inlet formed in the body, the inlet adapted to receive pressurized fluid from a pressure source, wherein the body expands or contracts responsive to the pressurized fluid received from the pressure source, and wherein, responsive to the expansion or contraction of the body, the mounting sleeve moves relative to the mounting bracket, thereby actuating the movable stem, characterized in that, the second end coupled to the second body portion comprises a second opening arranged to receive the second arm of the mounting assembly. a mounting assembly comprising a mounting bracket configured to be coupled to the bonnet of the shut-off valve and a mounting sleeve configured to be coupled to the movable stem of the shut-off valve, the mounting sleeve being movable relative to the mounting bracket, the mounting assembly further comprising a first arm coupled to the mounting bracket and a second arm coupled to and extending outward from the mounting sleeve; and an actuator configured to be operatively coupled to the shut-off valve via the mounting assembly, the actuator comprising: a body defined by a first body portion and a second body portion movable relative to the first body portion to expand or contract the body; a first end coupled to the first body portion and comprising a first opening arranged to receive the first arm of the mounting assembly; a second end coupled to the second body portion; and an inlet formed in the body, the inlet adapted to receive pressurized fluid from a pressure source, wherein the body expands or contracts responsive to the pressurized fluid received from the pressure source, and wherein, responsive to the expansion or contraction of the body, the mounting sleeve moves relative to the mounting bracket, thereby actuating the movable stem, a body defined by a first body portion and a second body portion movable relative to the first body portion to expand or contract the body; a first end coupled to the first body portion and comprising a first opening arranged to receive the first arm of the mounting assembly; a second end coupled to the second body portion; and an inlet formed in the body, the inlet adapted to receive pressurized fluid from a pressure source, wherein the body expands or contracts responsive to the pressurized fluid received from the pressure source, and wherein, responsive to the expansion or contraction of the body, the mounting sleeve moves relative to the mounting bracket, thereby actuating the movable stem,