My primary research focus is galaxy formation and evolution, which I study using high performance computer simulations with and without hydrodynamics. They have led to many key insights, including the demonstration that adding dissipative gas dynamics to simulations of CDM cosmological models leads to the formation of systems with (approximately) the sizes and masses of observed galaxies, the demonstration that the Lyman Alpha forest arises naturally from inhomogeneities in the diffuse intergalactic medium in CDM cosmology and that the Lyman Alpha forest is a powerful cosmological probe in its own right, prediction of abundant and highly clustered populations of high redshift galaxies, and prediction of luminosity and age-dependent galaxy bias in good agreement with observed trends.