This project focuses on measuring the velocity dispersions, masses and density profiles of the dark matter halos of galaxies via the dynamics of satellite galaxies, using publicly available redshift surveys. The sample will be selected paying close attention to the effects of interlopers, and the results of the analysis will be used to address a number of issues in galaxy formation, including halo mass versus Hubble type, mass-to-light ratio versus luminosity, radial dependence of kinematics, and alignment preferences. Interpretation will be aided by high-resolution computer simulations, addressing the validity of mass estimators and the impact of interlopers. The research will include undergraduate activities far beyond their usual education experience and with longer-term involvement.