A motion tracking system for tracking the motion of a subject in an environment, the system comprising: an imaging device adapted to be worn by the subject; an accelerometer adapted to be worn by the subject; and a processor configured to receive a series of images of the environment captured by the imaging device and to form, in dependence on the images an estimate of the subject's position in the environment, and to receive acceleration data from the accelerometer and to, form an estimate of the motion of a part of the subject's body carrying the accelerometer in dependence on the acceleration data and the estimated position.