Soybeans contain several physiologically active ingredients, such as soy phytosterol, soyasaponin, soy protein, and lecithin, and are therefore expected to express the functionalities of said ingredients. Among them, soy isoflavones have been studied in recent years for their various functions, including their obesity-preventing effect, blood glucose level reducing effect, osteoporosis and breast cancer risk reduction, and anti-oxidative effect, and several health promoting effects and disease preventing effects are expected. For example, it has been determined that soy isoflavones reduce body and fat weight in experiments in which mice were fed a diet containing soy isoflavones in studies on anti-obesity. Epidemiologic studies with humans have also shown that women who consume more soybeans have lower BMI than those who consume less. We previously found that soy isoflavones may have anti-obesity effects in myoblasts through the activation of transcriptional coactivator PGC-1β, which increases energy expenditure. In recent studies, a decrease in blood glucose level due to soy isoflavone was seen in an experiment in which diabetic model mice were fed a diet containing soy isoflavone. It has also been suggested that soy isoflavone intake may increase bone mineral density in postmenopausal women and reduce the risk of breast cancer. This review focuses on the actions of soy isoflavones known to date, including their anti-obesity and anti-diabetic effects, bone loss preventing effects, and cancer risk reduction effects, and introduces reports on the health promotion and disease prevention effects of soy isoflavones.