A robotic surgical instrument comprising a shaft, an articulated section and a drive mechanism. The articulated section extends from the shaft and terminates at its distal end in a tip. The tip has an attachment for an end effector. The drive mechanism drives the articulated section via flexible driving elements thereby altering the angular orientation of the tip relative to the shaft. The drive mechanism is controlled so as to always fully compress the articulated section along at least one extent by which the articulated section connects the tip and the shaft whilst driving the articulated section from any one configuration to any other configuration. The drive mechanism constrains movement of the articulated section so as to permit the tip to move with two degrees of rotational freedom and no degrees of translational freedom relative to the shaft.