This volume summarizes the evolution and physiology of GnRH molecules and receptors, and provides insight as to how social behavior influences cellular and molecular events in the brain from a comparative perspective. The chapters in this volume are divided into three major sections: Development and Cell Migration, GnRH Receptors, Physiology and Regulation. The review papers arose primarily from presentations made at the Second International Symposium on the Comparative Biology of GnRH, held in Penang, Malaysia, June 2-4, 2001; a satellite symposium in conjunction with the XIV International Congress of Comparative Endocrinology, Sorrento, Italy. In addition, leading neuroscientists doing cutting-edge research in the field of GnRH were invited as authors to make this volume a valuable reference.