Soybeans are greatly used in many foods, such as tofu, soy milk and yogurt, and it plays an important role as a functional food in the health and prevention of various diseases. Soybeans contain rich isoflavones including genistein, daidzein and glycitein, and both genistein and daidzein occur at higher concentrations. Isoflavones are structurally and functionally similar to 17ss-estradiol. Isoflavones are found to bind to estrogen receptors and the peroxisome proliferator-activated receptor (PPAR) family and aryl hydrocarbon receptors. Moreover, isoflavones affect activity of tyrosine kinases, histones, transcription factors, adenosine mono phosphate pathways, RNA expression and other intracellular regulators. Thus, isoflavones play a multifunctional role in cell regulation. This book focuses on recent advances in the food sources, biological activity and health benefits of genistein and daizen.