Isoflavones are predominantly found in legumes such as soya bean, chickpeas, beans, green peas, pistachios, peanuts, red clover, alfalfa, kodzu roots, other fruits, nuts, and other secondary products of legumes like tofu, soy milk, and so on. Among the entire natural sources, soybean contains nutritionally relevant amounts of isoflavones. The isoflavones are also known as phytoestrogen owing to the structural similarity with the mammalian estrogen and affinity toward estrogen receptors. Thus, it is suggested that the isoflavones might prevent hormone-dependent breast and prostate cancers by virtue of their potential estrogen-antagonist activity. It is also found to exert potential antioxidant and anti-inflammatory activities and thus used in gastric cancer. Its affinity toward the estrogenic receptor facilitates the target-specific activity, which further increases the potency of the isoflavones. On the basis of the source and chemical structures, different types of isoflavones are found in nature with slightly different behavior. In this chapter, we have discussed the occurrence, classification, mechanism of cancer prevention, and efficacy of dietary isoflavones in the treatment of various types of cancer.