Leptin (meaning thin) is attracting the attention of many scientists of the world recently. It is an adipocyte-derived protein hormone discovered in 1994. Human leptin gene is located on chromosome 7. It is mainly expressed in adipose tissue but also in skeletal muscle, stomach, placenta and mammary gland. Leptin play key role in food intake, energy balance, and adiposity as well as in immune and endocrine system. It acts as feedback loop to maintain the constant store of body fat. Leptin acts as an antiobesity hormone raising the potential of its use as antiobesity drugs. In future, leptin or its analogue may offer noble therapeutic approach for obesity or other leptin related disorders. This review focuses on current knowledge of leptin biology and the role of leptin in various physiological and pathophysiological states.