An ultra-high pressure pump, for use in a waterjet cutting apparatus, comprising: two pistons (50, 51), two cylinders (54, 55), and two high pressure seals (70, 71) between the inner ends of the cylinders and the pistons; and a drive mechanism for driving the pump; the drive mechanism comprising a servo motor (15, 19) comprising a hollow rotor and a stator coaxially mounted around the hollow rotor; an output shaft having ends; and a controller (82, 83, 84) coupled to the servo motor; each of the ends of the output shaft being coupled to a respective one of the pistons, each piston being adapted to complete reciprocal motion within associated cylinders thus defining two pumping chambers; the hollow rotor including drive means co-axially coupled to the output shaft whereby the drive means converts rotational movement of the hollow rotor to linear, reciprocal displacement of the output shaft to pressurize the fluid in the pumping chambers; the drive mechanism further comprising an encoder configured to measure movement of the hollow rotor or the output shaft and to send a feedback signal proportional to the movement to the controller, the encoder being coupled to the controller via a feedback loop; and characterized by the drive mechanism being a drive mechanism for driving the pump to deliver fluid at more than 345 MPa (50,000 psi); one end of the hollow rotor being supported by an annular bearing located between a housing and the hollow rotor; the output shaft comprising a screw supported by linear bearings (23, 24) each supported on elongate rails (25, 26); and the rails passing through the annular bearing. two pistons (50, 51), two cylinders (54, 55), and two high pressure seals (70, 71) between the inner ends of the cylinders and the pistons; and a drive mechanism for driving the pump; the drive mechanism comprising a servo motor (15, 19) comprising a hollow rotor and a stator coaxially mounted around the hollow rotor; an output shaft having ends; and a controller (82, 83, 84) coupled to the servo motor; a servo motor (15, 19) comprising a hollow rotor and a stator coaxially mounted around the hollow rotor; an output shaft having ends; and a controller (82, 83, 84) coupled to the servo motor; each of the ends of the output shaft being coupled to a respective one of the pistons, each piston being adapted to complete reciprocal motion within associated cylinders thus defining two pumping chambers; the hollow rotor including drive means co-axially coupled to the output shaft whereby the drive means converts rotational movement of the hollow rotor to linear, reciprocal displacement of the output shaft to pressurize the fluid in the pumping chambers; the drive mechanism further comprising an encoder configured to measure movement of the hollow rotor or the output shaft and to send a feedback signal proportional to the movement to the controller, the encoder being coupled to the controller via a feedback loop; and characterized by the drive mechanism being a drive mechanism for driving the pump to deliver fluid at more than 345 MPa (50,000 psi); one end of the hollow rotor being supported by an annular bearing located between a housing and the hollow rotor; the output shaft comprising a screw supported by linear bearings (23, 24) each supported on elongate rails (25, 26); and the rails passing through the annular bearing.