  I present a general discussion of the evolution and model-dependence of both
the linear and nonlinear power spectrum of density fluctuations. The features
of the linear power spectrum in cosmological models with cold dark matter (CDM)
and cold+hot dark matter (C+HDM) are reviewed, and useful analytical
approximations are summarized. Cosmological numerical simulation results are
then used to illustrate the process of nonlinear gravitational clustering and
to compute the nonlinear power spectrum. An analytical approximation relating
the linear and nonlinear power spectrum in C+HDM models is presented.
