  We review the role in physics of the Pin groups, double covers of the full
Lorentz group. Pin(1,3) is to O(1,3) what Spin(1,3) is to SO(1,3). The
existence of two Pin groups offers a classification of fermions based on their
properties under space or time reversal finer than the classification based on
their properties under orientation preserving Lorentz transformations --
provided one can design experiments that distinguish the two types of fermions.
Many promising experimental setups give, for one reason or another, identical
results for both types of fermions. Two notable positive results show that the
existence of two Pin groups is relevant to physics:
  1) In a neutrinoless double beta decay, the neutrino emitted and reabsorbed
in the course of the interaction can only be described in terms of Pin(3,1).
  2) If a space is topologically nontrivial, the vacuum expectation values of
Fermi currents defined on this space can be totally different when described in
terms of Pin(1,3) and Pin(3,1).
  Possibly more important than the two above predictions, the Pin groups
provide a simple framework for the study of fermions; they make possible clear
definitions of intrinsic parities and time reversal. A section on Pin groups in
arbitrary spacetime dimensions is included.
