  I review theoretical models of star formation and how they apply across the
stellar mass spectrum. Several distinct theories are under active study for
massive star formation, especially Turbulent Core Accretion, Competitive
Accretion and Protostellar Mergers, leading to distinct observational
predictions. These include the types of initial conditions, the structure of
infall envelopes, disks and outflows, and the relation of massive star
formation to star cluster formation. Even for Core Accretion models, there are
several major uncertainties related to the timescale of collapse, the relative
importance of different processes for preventing fragmentation in massive
cores, and the nature of disks and outflows. I end by discussing some recent
observational results that are helping to improve our understanding of these
processes.
