Melatonin (N-acetyl-5-methoxytryptamine) exists in plants, although it is commonly known as a neurohormone in animals. Indeed, the melatonin level is very high in some medicinal plants and changes developmental stage specifically, indicating that it plays specific physiological roles. Plant melatonin may play unique roles in plants as well as similar functions in animals. Furthermore, exogenously applied melatonin affects developmental processes during both vegetative and reproductive growth. In this study, current knowledge regarding plant melatonin is reviewed and its implications and problems are discussed.