The semiclassical approach of Onsager and Pippard has been very succes
sful in relating the electronic properties of a normal metal to its ba
nd structure and fermi surface. This paper extends the method to a sup
erconductor. A generalized London equation relating the supercurrent d
ensity to the vector potential is obtained, in terms of band and fermi
surface parameters and an energy gap which may be anisotropic. The re
sults allow an interpretation of measured penetration depths directly
in terms of the electronic and gap structures of the superconductor. T
hey lead to easy physical visualization of the origin of anisotropies
and other features of measured penetration depths.