  There are several interacting spin systems which have a gap in their spin
excitation spectrum. The gap does not occur due to anisotropies present in the
system but is quantum mechanical in origin. We give a brief overview on the
different types of spin gap (SG) antiferromagnets, the models proposed to
describe their physical properties and experimental realizations of such
systems. Our special focus is on exactly-solvable models and rigorous theories
which provide the correct physical picture for the novel phenomena exhibited by
SG systems
