While the cancer protective effect of soy-based diets has been the subject of numerous studies, the constituents of soy that may give rise to this effect remain elusive. Recent publications describing anticancer activity of crude and purified soybean saponins have sparked a renewed interest in these compounds. In this review, I summarize the epidemiological studies concerning the cancer protective effects of soy and the efforts to elucidate the constituents responsible for this effect. The recent reports of the anticancer activity of soy saponins is placed in context with reports of promising anticancer activity of structurally related non-dietary saponins from other legumes. While recent studies have demonstrated a direct effect of soy saponins on cancer cells, alternative mechanisms of cancer prevention by these agents are also discussed. It is concluded that the soy saponins may represent promising leads both in terms of elucidating the soy constituents involved in the cancer protective effect of soy as well as in the discovery of anticancer agents with novel mechanisms of action.