David Feldman, MD, is Emeritus Professor of Medicine (Active) at Stanford University School of Medicine where he has been on the faculty since 1974. He has been a full professor since 1984 and was chief of the Endocrinology Division for 10 years. His laboratory studies the role of steroid hormone receptors, particularly the vitamin D receptor, and its mechanism of action. His current major research focus is hormone-dependent cancer including breast cancer and prostate cancer and the pathways by which vitamin D inhibits cancer growth. Professor Feldman is actively involved in both basic science approaches to the anti-cancer actions of vitamin D as well as to clinical trials studying the use of vitamin D in breast and prostate cancer. Professor Feldman was recently honored with an award for a Career of Outstanding Contributions to Vitamin D Research. He has authored over 290 medical research articles, reviews, editorials, and book chapters. In addition to being a co-editor of all four editions of OSTEOPOROSIS, he is the editor-in-chief of Vitamin D, just published in its third edition.