In a universe dominated by dark matter and dark energy, galaxies and their host dark matter halos
grow primarily through mergers with other galaxies.  Having an accurate description of the mergers
of dark matter halos is therefore a key step in understanding galaxy formation and evolution.
I will give an overview of observational evidence for galaxy mergers and describe some recent
theoretical efforts in quantifying the rates of mergers and the impact on the growths of galaxies
and their central supermassive black holes.