AbstractMelatonin, a well-known neurohormone, was discovered in animal tissues in 1958. In plants, its presence was detected in 1995, and its biosynthesis, from tryptophan, seems to follow similar enzymatic steps in plant and animal cells, although with some very particular characteristics. Even though our knowledge of the biosynthetic enzymes of melatonin in plants is still taking its first steps, a greater plasticity has been observed in plant cells than in animal cells, as was to be expected. As they have been in animals, some catabolic products of the kynuric pathway have been determined in plants. Initially, melatonin was investigated to establish whether it had the same physiological function, as a chronoregulator, in plants as in animals. Its antioxidant properties have marked much of the research carried out into its possible physiological roles in both plants and animals. In plants, various studies have suggested more specific physiological actions, such as, for example, its roles as a plant growth regulator, an activator of rhizogenesis and a signal molecule/antioxidant in many plant stress situations. The melatonin levels that have been observed in different plant species vary widely - from picograms to micrograms per gram of tissue. The search for plant materials with high levels of melatonin has opened the door for its possible use as a nutraceutical for both humans and livestock. In addition, there has been great interest in the application of melatonin in agriculture to obtain more resistant crops and better harvests in adverse situations.