We consider the origins of the spin-dependent scattering which is beli
eved to he responsible for the giant magnetoresistance effect. We argu
e on the basis of first-principles calculations of the electronic stru
cture, magnetic moments, and electrical resistivities that this spin-d
ependent scattering originates largely from the tendency of spin-depen
dent atomic potentials to 'match' in one of the spin channels of magne
tic alloys or multilayers. We also argue that the matched channel will
tend to have a lower density of stares at the Fermi energy. Both of t
hese effects will contribute to the GMR. We argue that there is the po
tential for a very large GMR in an ideal system, but that spin-indepen
dent scattering possibly coming from misaligned spins near the interfa
ces prevents its observation.