This book presents an overview of the research advances and clinical challenges of the pineal gland. The topics analyzed cover a broad spectrum of functions played by the pineal gland and present new information in this area of research. These topics include: the physiology and pathology of skeletal and cardiac muscles; the beneficial actions of melatonin; n3 and n6 essential fatty acids, lipoxygenation and melatonin synthesis; the structure and functions of the fish pineal organ; therapeutical aspects of melatonin in bone marrow and thyroid-associated immune regulation; melatonin as a promising cytoprotective agent against ischemia-reperfusion injury in the liver; therapeutic approaches with melatonin; melatonin and neuroinflammation: encouraging findings vs. fundamental problems; melatonin and aging; pineal parenchymal tumor of intermediate differentiation; pineal glial cysts: features and clinical significance; melatonin as a modulator of radiosensibility and chemosensibility in cancer, circadian rhythm and melatonin associated with cancer development and treatment.