The spin degree of freedom can play an essential role in determining the
electrical transport properties of spin-polarized electron systems in metals or
semiconductors. In this article, I address the dependence of spin-subsystem
chemical potentials on accumulated spin-densities. I discuss both approaches
which can be used to measure this fundamental thermodynamic quantity and the
microscopic physics which determines its value in several different systems.Comment: 14 pages, 4 figures. Based on lecture given at the XVI Sitges
Conference, June 1999. Proceedings to be published by Springer-Verla