Melatonin (N-acetyl-5-methoxytryptamine) was first purified and characterized from the bovine pineal gland extract by Aron Lerner and co-workers in 1958. Since then, a plethora of information has piled up on its biosynthesis, metabolism, time-bound periodicity, physiological and patho-physiological functions, as well as its interactions with other endocrine or neuro-endocrine organs and tissues in the body. Melatonin has wide range of applications in physiology and biomedical fields. In recent years, a significant progress has been made in the understanding mechanism of its actions at the cellular and molecular levels. Consistent efforts have uncovered the mystery of this indoleamine, and demonstrated its role in regulation of a large as well as diverse body functions in different groups of animals in general, and in humans in particular. Current review, in commemoration of 50 years of discovery of melatonin, while revisiting the established dogmas, summarizes current information on biosynthesis, secretion, metabolism and molecular mechanism of action of melatonin at cellular level and highlights the recent research on its role in human physiology and clinical biology.