Genistein is a soybean isoflavone with diverse biological activities. It is a potent antioxidant, a specific inhibitor of protein tyrosine kinase, and a phytoestrogen. In recent years, increasing evidence has accumulated that this natural ingredient shows preventative and therapeutic effects for breast and prostate cancers, postmenopausal syndrome, osteoporosis, and cardiovascular diseases in animals and humans. In the past decade we have conducted a series of studies and demonstrated that genistein has significant antiphotocarcinogenic and antiphotoaging effects. Genistein substantially inhibits skin carcinogenesis and cutaneous aging induced by ultraviolet (UV) light in mice, and photodamage in humans. The mechanisms of action involve protection of oxidative and photodynamically damaged DNA, downregulation of UVB-activated signal transduction cascades, and antioxidant activities. In this article, we review the biological activities of genistein, as well as published and unpublished research from our laboratory. In addition, we discuss the potential application of genistein to clinical dermatology.