  We have calculated the nonlinear current of a number of single band s-wave
electron-phonon superconductors. Among issues considered were those of
dimensionality, strong electron-phonon coupling, impurities, and comparison
with BCS. For the case of two bands, particular attention is paid to the role
of anisotropy, the integration effects of the off-diagonal electron-phonon
interaction, as well as inter- and intraband impurities. For the specific case
of MgB2, we present results based on the known microscopic parameters of band
theory.
