Leptin, a recently described type-1 cytokine, is involved in cellular maturation and growth and appears to have a relationship to some obstetrical and gynecologic diseases. The MEDLINE database was accessed, and leptin-related articles published during the past 6 years were reviewed for their relevance to gynecologic and obstetrical diseases. The relationships between this cytokine and obesity, puberty, polycystic ovary syndrome, endometriosis, assisted fertility, and menopause are discussed. The role of leptin in fetal physiology and in normal and abnormal fetal growth as well as its role in diabetes, pregnancy, and pregnancy-induced hypertension are reviewed. Target Audience: Obstetricians & Gynecologists, Family Physicians Learning Objectives: After completion of this article, the reader will be able to describe leptin physiology, to outline the various relationships between leptin and the endocrine system, and to explain leptin physiology during pregnancy.