Melatonin is an indolamine present in animals and other organisms. It was not until 1993 that this compound was detected in plants. It is an indolic compound derived from the amino acid tryptophan. The existence of endogenous circadian rhythms in melatonin levels has been demonstrated and the data suggest a central role of this molecule in the day/night cycles in plants. One of the most studied actions is its effect on biotic and abiotic stresses in the plant, such as drought, extreme temperature, chemical pollution, UV radiation. It acts as a scavenger of reactive oxygen and reactive nitrogen species (ROS and RNS), improves the redox status of the cell and stabilizes biological membranes. Melatonin as plant regulator regulates the growth of the roots, shoots, activates the rhizogenesis and inhibits the senescence of the leaves. In all these processes, its quantity is important. The effect in micromolar concentrations can be exactly opposite to higher concentrations. It has been shown that melatonin increases the efficiency of photosynthesis by affecting photosystem II and allowing leaves to mantain a high capacity for CO2 in stress situations. Its biostimulatory role is also important for agriculture.