  Resistance of certain (conductive and otherwise isotropic) ferromagnets turns
out to exhibit anisotropy with respect to the direction of magnetisation:
R$_\parallel$ different from R$_\perp$ with reference to the electric current
direction. This century-old phenomenon is reviewed both from the perspective of
materials and physical mechanisms involved. More recently, this effect has also
been extended to antiferromagnets. This opens the possibility for industrial
applications reaching far beyond the current ones, e.g. hard drive read heads
or position sensors.
