Itinerant electrons moving in a metal urider the influence of an applied electric field suffer momentum-changing collisions from numerous causes: collision, with their own kind, with native or foreign ions, or with any other deviation from the periodicity of the ideel lattice structure. The subject of much recent research are those collisions which have left-right asymmetry; that is, the scattered electron has an unequal probability of passing to one side of the scatterer rather than to the other. Such Bsymmetric effects are most prominent in the case of ecatteriug by an ion which has a looalized magnetic moment and is in an applied magnetic field, although in principle any mattering centre having an attractive potential can give asymmetric scattering. This erticle gives an elementary and descriptive review of the microscopic processes leading to such scattering, and covers recent developments in their macroscopic manifestations.