Melatonin is a pleiotropic molecule with multiple functions in plants. Since the discovery of mela-tonin in plants, numerous studies have provided insight into the biosynthesis, catabolism, and physiological and biochemical functions of this important molecule. Here, we describe the bio-synthesis of melatonin from tryptophan, as well as its various degradation pathways in plants. The identification of a putative melatonin receptor in plants has led to the hypothesis that melatonin is a hormone involved in regulating plant growth, aerial organ development, root morphology, and the floral transition. The universal antioxidant ac-tivity of melatonin and its role in preserving chlorophyll might explain its anti-senescence ca-pacity in aging leaves. An impressive amount of research has focused on the role of melatonin in modulating postharvest fruit ripening by regu-lating the expression of ethylene-related genes. Recent evidence also indicated that melatonin functions in the plant's response to biotic stress, cooperating with other phytohormones and well-known molecules such as reactive oxygen species and nitric oxide. Finally, great progress has been made towards understanding how melatonin alle-viates the effects of various abiotic stresses, in-cluding salt, drought, extreme temperature, and heavy metal stress. Given its diverse roles, we propose that melatonin is a master regulator in plants.