  Gray models, which replace spectrally-resolved opacities with a wavelength
independent mean opacity, are currently seeing wide and diverse application. In
this brief review, we discuss both the history of gray techniques as well as
recent applications of gray models, with an emphasis on planetary atmospheres.
Methods and results for generating mean opacities are summarized. We present
examples where gray radiative transfer tools are incorporated into
three-dimensional atmospheric circulation models. Gray techniques are also
useful for problems in comparative climatology, and we inter-compare results
from several generalized gray models as applied to the computation of
convective fluxes in planetary atmospheres. Finally, we provide examples where
future progress can be made in the development of gray models.
