Abstract: A device is presented for quantitatively measuring and developing proprioceptive ability. The device is also useful for rehabilitating injuries, particularly those to the lower body. The device comprises a rigid platform affixed to an inelastic, unstable rocker assembly by means of a support. In one preferred embodiment the rocker assembly rests on the floor. In another embodiment, the rocker assembly rests on the surface of a contact bed. The user stands on the platform and, by shifting his/her center of gravity with respect to the unstable rocker assembly, brings the platform into approximate balance wherein the plane of the platform is substantially parallel to the floor within certain limits. The ability of the user to keep the plane of the platform within a predetermined range of inclination with respect to the horizontal, measured as a function of time, enables quantification of the increase in proprioceptive ability. The device also provides rehabilitative and conditioning exercise.