  Neutrinoless double beta decay violates lepton number by two units, a
positive observation therefore necessarily implies physics beyond the standard
model. Here, three possible contributions to neutrinoless double beta decay are
briefly reviewed: (a) The mass mechanism and its connection to neutrino
oscillations; (b) Left-right symmetric models and the lower limit on the
right-handed W boson mass; and (c) R-parity violating supersymmetry. In
addition, the recently published ``extended black box'' theorem is briefly
discussed. Combined with data from oscillation experiments this theorem
provides proof that the neutrinoless double beta decay amplitude must receive a
non-zero contribution from the mass mechanism, if neutrinos are indeed Majorana
particles.
