The identification and sequencing of the ob gene and its product, leptin, in late 1994 opened new insights in the study of the mechanisms controlling body weight and led to a surge of research activity. During this time, a considerable body of knowledge regarding leptin's actions has been accumulated and the field continues to expand rapidly. Currently there is particular interest in the interaction of leptin with other peripheral and neural mechanisms to regulate body weight, reproduction and immunological response. In this review, we attempt to place the current state of knowledge about leptin in the broader perspective of physiology, including its structural characteristics, receptors, binding proteins, signalling pathways, regulation of adipose tissue expression and production, secretion patterns, clearance mechanisms and functional effects. In addition, leptin's involvement in the pathophysiology of obesity, anorexia nervosa, diabetes mellitus, polycystic ovary syndrome, acquired immunodeficiency syndrome, cancer, nephropathy, thyroid disease, Cushing's syndrome and growth hormone deficiency will be reviewed.