  The formalism of classical and quantum mechanics on phase space leads to
symplectic and Heisenberg group representations, respectively. The Wigner
functions give a representation of the quantum system using classical
variables. The correspondence between the classical and the quantum criterion
of separability for the system is obtained in terms of these functions.
  Entanglement is generic and separability is special. Some applications are
discussed in commonly occuring examples and possibly in exotic systems.
