  Pair fluctuation theory has been used to study the crossover from the weak
coupling BCS theory to the strong coupling Bose Einstein Condensation. The
effect of fluctuations has been studied over the whole crossover regime. It has
been shown that the pair fluctuations are enhanced considerably in both two and
three dimensions and hence mean field theory is inadequate to study the
physical properties in this regime. A self consistent scheme for calculating
the pair susceptibility is given.
