  We examine the occurrence of Bose-Einstein condensation in both
nonrelativistic and relativistic systems with no self-interactions in a general
setting. A simple condition for the occurrence of Bose-Einstein condensation is
given. We show that condensation can occur only if $q\geq 3$, where $q$ is the
dimension associated with the continuous part of the eigenvalue spectrum of the
Hamiltonian for nonrelativistic systems or the spatial part of the Klein-Gordon
operator for relativistic systems. Furthermore we show that the criterion for
the appearance of the Meissner-Ochsenfeld effect is closely connected with that
for the appearance of Bose-Einstein condensation.
