Hormones are chemical messengers that are secreted directly into the blood, which transports them to organs and tissues of the body to exert their functions. There are several types of hormones that work on different aspects of body functions and processes. Estrogen, a steroid hormone, carries physiological messages to body organs and systems. These messages differ for each body organ and system. Estrogen sends messages to the uterus to grow and replace the lining that is shed during the previous menses. The three major naturally occurring estrogens in women are estrone (E1), estradiol (E2), and estriol (E3). Estradiol is the predominant estrogen during reproductive years both in terms of absolute serum levels as well as in terms of estrogenic activity. The present book deals with some phytoestrogens most widely used.