Book excerpt: The link between the pineal gland and cancer is a rapidly emerging research field due to promising experimental and clinical trials with melatonin. The pineal gland acts as a transducer of environmental light to regulate rhythmic processes, including reproductive function in seasonally breeding animals and the entrainment of circadian rhythms, such as the sleep-wake cycle, in man. This book elucidates the physiological significance of the pineal gland and surveys phenomena and mechanisms of pineal - tumor interaction at the neuroendocrine, neuroimmune, neural, and molecular levels. Yet unidentified low-molecular-weight pineal substances with tumor-inhibiting capacity, a possible involvement of melatonin in electromagnetic field effects on cancer, and the oncotherapeutic potential of melatonin are also addressed. The encouraging results should incite further research to elucidate the exact nature of the link between the pineal gland and cancer for the benefit of patients.