This volume brings together contributions from basic scientists, epidemiologists, and practicing clinicians whose expertise or clinical practice involves the study or administration of gonadal hormones. Our current knowledge of the biology of gonadal hormones is assessed, and the question is explored of the gaps in knowledge that need to be addressed through additional research. Other questions explored include the following: What is the appropriate population to study? What is the therapeutic window for hormone therapy and/or estrogen therapy? What are the appropriate formulations of HT and ET? What should be considered the most appropriate design for clinical trials that address the efficacy of HT/ET? What are appropriate surrogate markers of neurological decline/impairment? What are the best in vitro and in vivo models to address the efficacy of postmenopausal HT or ET? NOTE: Annals volumes are available for sale as individual books or as a journal. For information on institutional journal subscriptions, please visit www.blackwellpublishing.com/nyas. ACADEMY MEMBERS: Please contact the New York Academy of Sciences directly to place your order (www.nyas.org). Members of the New York Academy of Science receive full-text access to the Annals online and discounts on print volumes. Please visit http://www.nyas.org/MemberCenter/Join.aspx for more information about becoming a member.