  We consider the effect of nonmagnetic and magnetic impurities on the
superheating field $H_s$ in a type-II superconductor. We solved the Eilenberger
equations, which take into account the nonlinear pairbreaking of Meissner
screening currents, and calculated $H_s(T)$ for arbitrary temperatures and
impurity concentrations in a single-band s-wave superconductor with a large
Ginzburg-Landau parameter. At low temperatures nonmagnetic impurities suppress
a weak maximum in $H_s(T)$ which has been predicted for the clean limit,
resulting instead in a maximum of $H_s$ as a function of impurity concentration
in a moderately clean limit. It is shown that nonmagnetic impurities weakly
affect $H_s$ even in the dirty limit, while magnetic impurities suppress both
$H_s$ and the critical temperature $T_c$. The density of quasiparticles states
$N(\epsilon)$ is strongly affected by an interplay of impurity scattering and
current pairbreaking. We show that a clean superconductor at $H=H_s$ is in a
gapless state, but a quasiparticle gap $\epsilon_g$ in $N(\epsilon)$ at $H=H_s$
appears as the concentration of nonmagnetic impurities increases. As the
nonmagnetic scattering rate $\alpha$ increases above $\alpha_c=0.36$, the
quasiparticle gap $\epsilon_g(\alpha)$ at $H=H_s$ increases, approaching
$\epsilon_g\approx 0.32\Delta_0$ in the dirty limit $\alpha\gg 1$, where
$\Delta_0$ is the superconducting gap parameter at zero field. The effects of
impurities on $H_s$ can be essential for the nonlinear surface resistance and
superconductivity breakdown by strong RF fields.
