This book chapter discusses a brief history of the caffeinated beverages coffee, tea and energy drinks, as well as other sources of caffeine; trends in consumption of caffeine; caffeine metabolism; the physiological effects of caffeine on different organ systems and on reproduction, physical performance and sleep; the neurophysiological and behavioural actions of methylxanthines; the modes of action of caffeine within the central nervous system; caffeine intoxication; and, caffeine and addiction.