This 8-chapter book is written by experts from China, Mexico, Portugal, United Kingdom and the United States of America. The book is focused on estrogen receptor structure, functions and clinical significance. Though we are unable to address all aspects of the estrogen receptors, we have analysed and discussed estrogen receptors or their isoforms in some major estrogen receptor-related disorders including cancers, endocrine diseases, respiratory diseases and obesity. The information provided by this book should aid our understanding the role of estrogen receptors in human diseases. This book will be of great value not only for clinicians interested in estrogen receptor-related diseases but also for basic scientists working in the field of estrogen receptors.