Melatonin has revealed itself to be a pleiotropic and multitasking molecule. The mechanisms that control its synthesis and the biological clock processes that modulate the circadian production of melatonin in the pineal gland have been well-characterized. A feature that characterizes melatonin is the variety of mechanisms it employs to modulate the physiology and molecular biology of cells. Research has implicated the pineal gland and melatonin in the processes of both aging and age-related diseases. The decline in the production of melatonin with age is thought to contribute to immunosenescence and potential development of neoplastic diseases. Melatonin has been shown to inhibit growth of different tumors under both in vitro and in vivo conditions. There is evidence that the administration of melatonin alone or in combination with interleukin-2 in conjunction with chemoradiotherapy and/or supportive care in cancer patients with advanced solid tumors, has been associated with improved outcomes of tumor regression and survival. Moreover, chemotherapy has been shown to be better tolerated in patients treated with melatonin.