This chapter describes the pharmacological properties of caffeine underlying its mechanism of action and details some of the neurophysiological properties of the methylxanthine. It also describes several biochemical mechanisms of action of caffeine. If the consumption of caffeine is blocking the actions of endogenous adenosine at its receptors, it implies that adenosine must be present at levels high enough to activate adenosine receptors under basal conditions. The effects of caffeine on locomotor activity have been reported for a long time and extensively studied. Low doses of caffeine act positively on mood; subjects ingesting 20 to 200 mg of caffeine report that they feel energetic, imaginative, efficient, self-confident, alert, able to concentrate, and motivated to work. The effects of caffeine on spontaneous locomotor activity are biphasic. The property of caffeine to increase wakefulness is one of the reasons why people consume daily caffeine and also represents one of the reasons why certain people limit ingestion of caffeine-containing drinks.