Leptin (from the Greek leptos, meaning thin) is a 167-amino acid protein hormone with important effects in regulating body weight, metabolism and reproductive function. Leptin, an adipose tissue derived hormone relays information about the peripheral energy reserve and their availability to the brain. This review article is on understanding the physiology of leptin and its role in energy homeostasis thereby controlling obesity, neuroendocrine function, energy metabolism and insights into emerging clinical applications. It also addresses the possible therapeutic uses of leptin in humans.