  This article concerns the formation and structure of dark matter halos,
including (1) their radial density profiles, (2) their abundance, and (3) their
merger rates. The last topic may be relevant to the nature of the small,
bright, high-redshift galaxies discovered by the Lyman break technique. (1)
Study of a statistical sample of galaxy-mass dark halos in high-resolution
Adaptive Refinement Tree simulations shows that they have a central density
profile p(r) \propto 1/r^g with g \approx 0.2, in agreement with data on
dark-matter-dominated disk galaxies. We present recent, higher resolution
results on this. (2) Another important new result is that the Press-Schechter
approximation predicts about twice as many galaxy-mass halos at z=0 as are
present in large dissipationless N-body simulations; more generally, PS
overpredicts the abundance of M ~< 0.1M_* halos at all redshifts. (3) Finally,
we discuss the assembly of these halos, in particular the merger rate of
(sub-)halos at high redshift and the distribution of the starbursts that these
mergers are likely to trigger. If most of the Lyman-break galaxies are such
starbursts, this perhaps resolves the apparent paradox that these galaxies
appear to cluster like massive halos (~10^{12} M_\odot), while their relatively
low linewidths and their spectral energy distributions suggest that they have
relatively low mass (few X 10^{10} M_\odot) and young ages (few X 10^8 yr). It
also predicts much more star formation at high redshift in CDM-type
hierarchical models for structure formation than if only quiescent star
formation is included.
