Estrogen receptors (ER) are emerging as important molecules involved in the variety of physiological and pathological activities. There are two classic estrogen receptors: alpha and beta. Different cells and tissues have shown divergent responses to these two estrogen receptors. While we are starting to understand the interaction between estrogen receptors and its ligands contributes to the development of various diseases, the discovery of sub-isoforms of estrogen receptor alpha and beta has further complicated the mechanism of estrogen receptors. Nevertheless, continuing efforts in the study of estrogen receptors have helped us to define their roles in diseases and to develop novel therapies against these disorders. The 11 chapters in this book are the excellent evidence to summarize our efforts in this direction. This book should be of great value to not only clinicians and medical students interested in estrogen/ER-related diseases but also basic scientists working in the field of estrogens/ERs.