  We investigate the effects of material anisotropy on the superheating field
of layered superconductors. We provide an intuitive argument both for the
existence of a superheating field, and its dependence on anisotropy, for
$\kappa = \lambda / \xi$ (the ratio of magnetic to superconducting healing
lengths) both large and small. On the one hand, the combination of our
estimates with published results using a two-gap model for MgB${}_2$ suggests
high anisotropy of the superheating field near zero temperature. On the other
hand, within Ginzburg-Landau theory for a single gap, we see that the
superheating field shows significant anisotropy only when the crystal
anisotropy is large and the Ginzburg-Landau parameter $\kappa$ is small. We
then conclude that only small anisotropies in the superheating field are
expected for typical unconventional superconductors near the critical
temperature. Using a generalized form of Ginzburg Landau theory, we do a
quantitative calculation for the anisotropic superheating field by mapping the
problem to the isotropic case, and present a phase diagram in terms of
anisotropy and $\kappa$, showing type I, type II, or mixed behavior (within
Ginzburg-Landau theory), and regions where each asymptotic solution is
expected. We estimate anisotropies for a number of different materials, and
discuss the importance of these results for radio-frequency cavities for
particle accelerators.
