  It has long been recognized that tight relations link the mass, size, and
characteristic velocity of galaxies. These scaling laws reflect the way in
which baryons populate, cool, and settle at the center of their host dark
matter halos; the angular momentum they retain in the assembly process; as well
as the radial distribution and mass scalings of the dark matter halos. There
has been steady progress in our understanding of these processes in recent
years, mainly as sophisticated N-body and hydrodynamical simulation techniques
have enabled the numerical realization of galaxy models of ever increasing
complexity, realism, and appeal. These simulations have now clarified the
origin of these galaxy scaling laws in a universe dominated by cold dark
matter: these relations arise from the tight (but highly non-linear) relations
between (i) galaxy mass and halo mass, (ii) galaxy size and halo characteristic
radius; and (iii) from the self-similar mass nature of cold dark matter halo
mass profiles. The excellent agreement between simulated and observed galaxy
scaling laws is a resounding success for the LCDM cosmogony on the highly
non-linear scales of individual galaxies.
