  Cold + Hot Dark Matter (CHDM) is perhaps the best theory of cosmic structure
formation {\it if} the cosmological matter density is near critical (i.e.,
$\Omega_0 \approx 1$) and {\it if} the expansion rate is not too large (i.e. $h
\equiv H_0/(100 \kmsMpc) \lsim 0.6$). I discuss CHDM together with its chief
competitor among CDM variants, low-$\Omega_0$ CDM with a cosmological constant
(\lcdm). \lcdm\ with $\Omega_0 \sim 0.3$ has the possible virtue of allowing a
higher expansion rate $H_0$ for a given cosmic age $t_0$, but the defect of
predicting too much fluctuation power on small scales. Also, except for the
$H_0-t_0$ problem, there is not a shred of evidence in favor of a nonzero
cosmological constant, only increasingly stringent observational upper bounds
on it. CHDM has less power on small scales, in good agreement with data,
although it remains to be seen whether it predicts early enough galaxy
formation to be compatible with the latest high-redshift data. CHDM has the
advantage among $\Omega=1$ CDM-type models of requiring little or no tilt,
which appears to be an advantage in fitting recent small-angle cosmic microwave
background anisotropy data. The presence of a hot component that clusters less
than cold dark matter lowers the effective $\Omega_0$ that would be measured on
small scales, which appears to be in accord with observations, and it may also
avoid the discrepancy between the high central density of dark matter halos
from CDM simulations compared to evidence from rotation curves of dwarf spiral
galaxies.
