Genistein (4',5,7-trihydroxyisoflavone) is a common precursor in the biosynthesis of antimicrobial phytoalexins and phytoanticipins in legumes, and an important nutraceutical molecule found in soybean seeds. Genistein is a phytoestrogen with a wide variety of pharmacological effects in animal cells, including tyrosine kinase inhibition, and dietary genistein ingestion has been linked, through epidemiological and animal model studies, with a range of potential health beneficial effects. These include chemoprevention of breast and prostate cancers, cardiovascular disease and post-menopausal ailments. In spite of an extensive literature on the effects of dietary genistein, questions still exist as to its potential overall benefits as a component of the human diet. Genistein can be synthesized chemically via the deoxybenzoin or chalcone route. Genistein is synthesized in plants from the flavanone naringenin by a novel ring migration reaction catalyzed by the cytochrome P450 enzyme isoflavone synthase (IFS). IFS genes have recently been cloned from a number of plant species, and production of genistein can be now achieved in non-legumes by recombinant DNA approaches.