The agricultural economy of India depends on the regularity of the summer monsoon rains although this climatic phenomenon exhibits a natural variability which often brings disaster. There exists much literature which discusses the physical mechanisms of the monsoon in its general aspects and some attention has been given to explanations of the variability of its main characteristics. Global circulation models afford an opportunity to examine the role of each of many physical components while allowing for interaction between the planetary circulation, sometimes even the oceanic circulation, and the monsoonal one over South Asia. Experiments to date have investigated the influence of Arabian Sea and Indian Ocean surface temperature anomalies, the influence of the Himalayan Mountains, of surface albedo and of the relative importance of sensible versus latent heating in maintaining the summertime regime. Armed with a greater understanding of the causes of monsoon variability, researchers will perhaps be able to begin developing techniques for seasonal forecasting.