In 1928 Bohr declared that it is impossible to use a Stern-Gerlach magnet to filter beams of electrons into their separate spin components. A proof of this assertion was presented by Pauli at the 1932 Solvay Conference on Magnetism. Yet recent experiments done by Dehmelt's group in Washington seem to cast doubt on Bohr's edict. This problem will be analyzed, and a counter-example to Pauli's proof presented. A new type of electron spin filter, recently demonstrated in our laboratory, will also be discussed.