  We present new results on the angular momentum evolution of dark matter
halos. Halos, from N-body simulations, are classified according to their mass
growth histories into two categories: the accretion category contains halos
whose mass has varied continuously and smoothly, while the merger category
contains halos which have undergone sudden and significant mass variations
(greater than 1/3 of their initial mass per event). We observe for the growth
of angular momentum that it continues well into the nonlinear regime, that it
is correlated with mass growth and that it occurs in two phases: a fast
followed by a slow phase. The spin parameter decreases with time for the
accretion catalog whereas slightly increases for the merger catalog, but does
not change when two catalogs are considered together, and furthermore it does
not depend on halo mass or cosmological model. We also develop a simple model
for the formation of a disk galaxy similar to Milky Way, and conclude that our
own halo must have captured satellites in order to acquire the required angular
momentum and to achieve most of the disk around z ~ 1.6. Angular momentum
distribution indicates that at z ~ 1.6 only 22% of the halos have angular
momentum of magnitude comparable to that of disk galaxies in the mass range of
$(0.1-5)*10^{11}$ of solar mass, clearly insufficient to explain the present
observed abundance of these objects.
