My research interests are centered on observational and theoretical studies of the structure and evolution of both star clusters and individual galaxies. Currently I am using imaging obtained with the Hubble Space Telescope to measure the systemic proper motions of the dwarf spheroidal satellite galaxies of our own Milky Way Galaxy. These motions yield the space velocity of each galaxy and, hence, its orbit around the Milky Way. Our goals are to use this information to determine if the dwarf spheroidals move together in coherent streams in the halo of our Galaxy, to constrain the importance of the Galactic tidal force in determining the structure and star-formation history of the dwarf spheroidals, and to compare the observed orbits with the predictions of models for the formation of our Galaxy. We also use the motions of the dwarf spheroidals to constrain the mass of the Milky Way.