A robot joint includes a casing, a motor assembly including a stator and a rotor that are arranged within the casing, and a harmonic drive received, at least in part, in the rotor. The harmonic drive includes a circular spline, a wave generator fixed to the rotor, and a flex spline. The circular spline is arranged around and engaged with the flex spline. The wave generator is received in the flex spline and configured to drive the flex spline to rotate with respect to the circular spline. The robot joint further includes an output shaft fixed to the flex spline.