Diet has been implicated to play an important role in cancers. Epidemiological studies have revealed that Asians, who consume a traditional diet high in soy products, have relatively low incidences of breast and prostate cancers, while the incidences are much higher in the Western world. Asians who immigrate to the United States and adopt a Western diet are at higher risks of breast and prostate cancers. Soy isoflavones have received much attention as dietary components having an important role in reducing breast and prostate cancers. Genistein, one of the predominant soy isoflavones, has been shown to inhibit the growth of cancer cells through the modulation of genes that are related to the homeostatic control of cell cycle and apoptosis. It has been found that genistein inhibits the activation of the nuclear transcription factor, NF-kappaB and Akt signaling pathway, both of which are known to maintain a balance between cell survival and programmed cell death (apoptosis). Genistein is known to have anti-oxidant property, and commonly known as phytoestrogen, which targets estrogen and androgen-mediated signaling pathway in the processes of carcinogenesis. Moreover, genistein is also found to be a potent inhibitor of angiogenesis and metastasis. Hence, significant advances have been made, both by in vitro and in vivo studies showing that genistein is a promising agent for cancer chemoprevention and/or treatment.