A high pressure intensifier system comprises a high pressure intensifier (3) for receiving hydraulic fluid from an input (1) and providing the fluid to an output (8) at a higher pressure than at the input; means (7) for monitoring the pressure of hydraulic fluid provided at the output; and control means (2, 9) for controlling the supply of hydraulic fluid from the input for maintaining the pressure of hydraulic fluid provided at the output at substantially a predetermined value.