Melatonin, N-acetyl-5-methoxytryptamine, is a molecule with diverse physiological functions. This neuro-hormone affects reproductive performance in a wide variety of species. In most animals, but not exclusively all, melatonin has an antigonadotrophic effect. The seasonal changes in the number of hours per day that melatonin is secreted mediate the temporal coupling of reproductive activity to seasonal changes in day-length. These observations stimulated a search for a role for the pineal gland and melatonin in human reproduction. Clinical experience related to this issue has yielded inconclusive and sometimes conflicting results. This article reviews the current available evidence concerning the effects of melatonin on human reproductive processes (e.g., puberty, ovulation, pregnancy, and fertility). Possible reasons for the vagueness and elusiveness of the clinical effects are discussed.