Melatonin (N-acetyl-5-methoxytryptamine), a well-known animal hormone, was discovered in plants in 1995 but very little research into it has been carried out since. It is present in different parts of all the plant species studied, including leaves, stems, roots, fruits and seeds. This brief review will attempt to provide an overview of melatonin (its discovery, presence and functions in different organisms, biosynthetic route, etc.) and to compile a practically complete bibliography on this compound in plants. The common biosynthetic pathways shared by the auxin, indole-3-acetic, and melatonin suggest a possible coordinated regulation in plants. More specifically, our knowledge to date of the role of melatonin in the vegetative and reproductive physiology of plants is presented in detail. The most interesting aspects for future physiological studies are presented.