Descriptions of the pineal gland date back to antiquity, but its functions in humans are still poorly understood. In both diurnal and nocturnal vertebrates, its main product, the hormone melatonin, is synthesized and released in rhythmic fashion, during the dark portion of the day-night cycle. Melatonin production is controlled by an endogenous circadian timing system and is also suppressed by light. In lower vertebrates, the pineal gland is photosensitive, and is the site of a self-sustaining circadian clock. In mammals, including humans, the gland has lost direct photosensitivity, but responds to light via a multisynaptic pathway that includes a subset of retinal ganglion cells containing the newly discovered photopigment, melanopsin. The mammalian pineal also shows circadian oscillations, but these damp out within a few days in the absence of input from the primary circadian pacemaker in the suprachiasmatic nuclei (SCN). The duration of the nocturnal melatonin secretory episode increases with nighttime duration, thereby providing an internal calendar that regulates seasonal cycles in reproduction and other functions in photoperiodic species. Although humans are not considered photoperiodic, the occurrence of seasonal affective disorder (SAD) and its successful treatment with light suggest that they have retained some photoperiodic responsiveness. In humans, exogenous melatonin has a soporific effect, but only when administered during the day or early evening, when endogenous levels are low. Some types of primary insomnia have been attributed to diminished melatonin production, particularly in the elderly, but evidence of a causal link is still inconclusive. Melatonin administration also has mild hypothermic and hypotensive effects. A role for the pineal in human reproduction was initially hypothesized on the basis of clinical observations on the effects of pineal tumors on sexual development. More recent data showing an association between endogenous melatonin levels and the onset of puberty, as well as observations of elevated melatonin levels in both men and women with hypogonadism and/or infertility are consistent with such a hypothesis, but a regulatory role of melatonin has yet to be established conclusively. A rapidly expanding literature attests to the involvement of melatonin in immune function, with high levels promoting and low levels suppressing a number of immune system parameters. The detection of melatonin receptors in various lymphoid organs and in lymphocytes suggests multiple mechanisms of action. Melatonin has been shown to be a powerful antioxidant, and has oncostatic properties as well, both direct and indirect, the latter mediated by its effects on reproductive hormones. Finally, there are reports of abnormal daily melatonin profiles in a number of psychiatric and neurological disorders, but the significance of such abnormalities is far from clear.