It is surprising that the current stellar mass of a galaxy contains all the information necessary to describe its past evolution. As such, the stellar mass tells us the star formation and metallicity history, formation epoch and environment of a galaxy. It is therefore surprising that galaxies are a one-parameter family... almost; some second order differences among galaxies still exits. To explain why galaxies are a one-parameter family remains one of the challenges of galaxy formation theories. In this talk I will describe new theoretical investigations that have yield new insights into the evolution and formation of galaxies. In particular, I will describe new theoretical tools to characterize the environment of galaxies and a modified halo model that have given us clues to understand why mass determines the life of a galaxy.