 Measurement of the proper motions of a large sample of stars in a distant stellar system, like a globular cluster, can be used to compute the cluster's total mass via the Leonard-Merritt mass estimator.Coupled with measurements of the stars' radial velocities, proper motions can be used to compute the distance to the cluster.Stellar proper motions have been used to infer the presence of a super-massive black hole at the center of the Milky Way.