A method of monitoring movement of a robotic arm, the robotic arm being arranged to be moved by an actuator, the method comprising: determining an expected robotic arm condition based on a known robotic condition and a torque applied to the robotic arm by the actuator; measuring an actual robotic arm condition during movement of the arm caused by the applied torque; and determining whether a collision has occurred by comparing the actual robotic arm condition with the expected robotic arm condition and generating a collision signal if a difference between the actual robotic arm condition and the expected robotic arm condition exceeds a threshold.