A hydraulic intensifier comprising: a first piston which is reciprocable in a first cylinder; a second piston which is reciprocable in a second cylinder; a cylindrical member joining the pistons so that each of them has a first face (6 or 6') which has a greater surface area than its second, opposite face (7 or 7') as a result of said cylindrical member, the first face of each of the pistons being at a respective low pressure side and the second face of each of the pistons being at a respective high pressure side; first and second inputs (19, 19') for supplying low pressure hydraulic fluid to respective ones of the low pressure sides; an output for high pressure hydraulic fluid from the high pressure sides; first and second solenoid operated pilot valves (8, 8') for controlling the supply of low pressure hydraulic fluid to respective ones of the inputs; electronic means arranged for operating the pilot valves for supplying low pressure hydraulic fluid alternately to the inputs; and coupling means (20, 20', 21, 21') between the first and second pistons, characterised in that: the coupling means is such that, if low pressure fluid is applied to one of said low pressure sides, said low pressure fluid also flows from said low pressure side through the coupling means to the high pressure side of the other of the pistons, the coupling means comprising a first passageway, between the low pressure side of the first piston and the high pressure side of the second piston, and a second passageway, between the low pressure side of the second piston and the high pressure side of the first piston, each of the passageways being provided with a respective non-return valve (21 or 21') for permitting flow from the low pressure side to the high pressure side. a first piston which is reciprocable in a first cylinder; a second piston which is reciprocable in a second cylinder; a cylindrical member joining the pistons so that each of them has a first face (6 or 6') which has a greater surface area than its second, opposite face (7 or 7') as a result of said cylindrical member, the first face of each of the pistons being at a respective low pressure side and the second face of each of the pistons being at a respective high pressure side; first and second inputs (19, 19') for supplying low pressure hydraulic fluid to respective ones of the low pressure sides; an output for high pressure hydraulic fluid from the high pressure sides; first and second solenoid operated pilot valves (8, 8') for controlling the supply of low pressure hydraulic fluid to respective ones of the inputs; electronic means arranged for operating the pilot valves for supplying low pressure hydraulic fluid alternately to the inputs; and coupling means (20, 20', 21, 21') between the first and second pistons, characterised in that: the coupling means is such that, if low pressure fluid is applied to one of said low pressure sides, said low pressure fluid also flows from said low pressure side through the coupling means to the high pressure side of the other of the pistons, the coupling means comprising a first passageway, between the low pressure side of the first piston and the high pressure side of the second piston, and a second passageway, between the low pressure side of the second piston and the high pressure side of the first piston, each of the passageways being provided with a respective non-return valve (21 or 21') for permitting flow from the low pressure side to the high pressure side. the coupling means is such that, if low pressure fluid is applied to one of said low pressure sides, said low pressure fluid also flows from said low pressure side through the coupling means to the high pressure side of the other of the pistons, the coupling means comprising a first passageway, between the low pressure side of the first piston and the high pressure side of the second piston, and a second passageway, between the low pressure side of the second piston and the high pressure side of the first piston, each of the passageways being provided with a respective non-return valve (21 or 21') for permitting flow from the low pressure side to the high pressure side.