Caffeine is the most widely consumed drug in the world, most commonly from the beverages coffee, tea and soda. An estimated 80% of the world's population consumes a caffeine-containing substance daily. A typical 8-ounce (240-ml) cup of instant coffee contains about 100 mg of caffeine - about twice as much as a cup of tea or a 12-ounce (360-ml) can/bottle of soda. A 30-gram chocolate bar might contain as much caffeine as half a cup of tea. More than 99% of orally ingested caffeine is absorbed -- with peak plasma levels obtained in 15 to 45 minutes. Caffeine is soluble in both water and oil and can readily cross the blood-brain barrier. Caffeine potentially has pharmacological actions other than blockage of adenosine receptors, but it requires 20 times as much caffeine to inhibit phosphodiesterase, 40 times as much caffeine to block GABAA receptors and 100 times as much caffeine to mobilise intracellular calcium as is required to block adenosine receptors. Caffeine acts primarily by the direct action of blocking adenosine receptors and by the indirect action upon the receptors for neurotransmitters. This book presents important research on this fascinating and relevant field of research.