  We consider spin transport and spin relaxation in superconductors using the
quasiclassical theory of superconductivity. We include spin relaxation due to
spin-orbit interaction as well as magnetic impurities, and show that the energy
dependence of the spin-flip rate is different for these two mechanisms. In
ferromagnet-superconductor-ferromagnet systems made of Co and Al, interface
resistances can be small compared to bulk resistances. This simplifies the
description of transport in Co/Al/Co spin valves, for which we numerically
calculate the temperature and Al length dependence of the magnetoresistance.
