Abstract: A drive unit for a surgical robot arm, the robot arm being configured to engage a robotic surgical instrument, the drive unit comprising a plurality of drive interface elements, each drive interface element having a longitudinal axis; a plurality of actuators configured to drive the plurality of drive interface elements, each actuator of the plurality of actuators being configured to drive one of the plurality of drive interface elements so as to cause that drive interface element to be displaced along its longitudinal axis in a first direction, wherein the drive unit is configured such that, when the surgical robot arm engages the robotic surgical instrument, the longitudinal axis of each drive interface element is aligned with a longitudinal axis of a respective instrument interface element in the instrument and each drive interface element is configured such that the displacement of said drive interface element along its longitudinal axis in the first direction causes a displacement of the respective instr