A blowout preventer which comprises: an elongated BOP housing; a BOP bore which extends axially through the BOP housing; and a pair of ram assemblies (22, 24) which are mounted on diametrically opposite sides of the BOP housing, each ram assembly (22, 24) comprising: a cylinder housing (34, 36) which comprises a recess that extends generally transversely relative to the BOP bore from a radially inner end of the cylinder housing (34, 36) to a transverse first wall located at a radially outer end of the cylinder housing (34, 36); a cylinder-ram (48, 50) which is slidably received in the recess and which comprises a cavity that extends from a radially outer end of the cylinder-ram (48, 50) to a transverse second wall located at a radially inner end of the cylinder-ram (48, 50); characterized in that each ram assembly (22, 24) comprises: a back plate which is connected to or formed integrally with the outer end of the cylinder-ram (48, 50), the cylinder-ram (48, 50) being sealed to the cylinder housing proximate the back plate; a piston head which is positioned in the cavity between the second wall and the back plate and is sealed to the cylinder-ram (48, 50), the piston head being connected to the cylinder housing; a first piston chamber which is defined between the back plate and the first wall; and a second piston chamber which is defined between the piston head and the second wall; wherein application of hydraulic pressure to the first and second piston chambers (116, 118) will move the cylinder ram (48, 50) from an open position in which the cylinder-ram (48, 50) is retracted from the BOP bore to a closed position in which the cylinder-ram (48, 50) extends across the BOP bore to approximately the centerline of the BOP bore. an elongated BOP housing; a BOP bore which extends axially through the BOP housing; and a pair of ram assemblies (22, 24) which are mounted on diametrically opposite sides of the BOP housing, each ram assembly (22, 24) comprising: a cylinder housing (34, 36) which comprises a recess that extends generally transversely relative to the BOP bore from a radially inner end of the cylinder housing (34, 36) to a transverse first wall located at a radially outer end of the cylinder housing (34, 36); a cylinder-ram (48, 50) which is slidably received in the recess and which comprises a cavity that extends from a radially outer end of the cylinder-ram (48, 50) to a transverse second wall located at a radially inner end of the cylinder-ram (48, 50); a cylinder housing (34, 36) which comprises a recess that extends generally transversely relative to the BOP bore from a radially inner end of the cylinder housing (34, 36) to a transverse first wall located at a radially outer end of the cylinder housing (34, 36); a cylinder-ram (48, 50) which is slidably received in the recess and which comprises a cavity that extends from a radially outer end of the cylinder-ram (48, 50) to a transverse second wall located at a radially inner end of the cylinder-ram (48, 50); a back plate which is connected to or formed integrally with the outer end of the cylinder-ram (48, 50), the cylinder-ram (48, 50) being sealed to the cylinder housing proximate the back plate; a piston head which is positioned in the cavity between the second wall and the back plate and is sealed to the cylinder-ram (48, 50), the piston head being connected to the cylinder housing; a first piston chamber which is defined between the back plate and the first wall; and a second piston chamber which is defined between the piston head and the second wall; wherein application of hydraulic pressure to the first and second piston chambers (116, 118) will move the cylinder ram (48, 50) from an open position in which the cylinder-ram (48, 50) is retracted from the BOP bore to a closed position in which the cylinder-ram (48, 50) extends across the BOP bore to approximately the centerline of the BOP bore.