Some plants from the tropic belt synthesize caffeine, starting with the purine base xanthosine, and leaves and seeds are particularly effective at caffeine biosynthesis. Communities living alongside these plants have traditionally used them as constituents for beverages or for masticatory consumption. In the modern age, the value of caffeine plants reached the nontropical regions, and caffeine products have become some of the most important commodities worldwide; coffee, tea, and cocoa are discussed in articles within this encyclopedia. The planar structure of caffeine's parent skeleton, purine, with partial charges, allows molecular interactions in aqueous solution, with a preference toward dimerization. In parallel, heteroassociation with molecules of similar geometry but different chemistry in the cellular environment leads to complexes that produce the specific patterns of stimulant behavior that characterize different plant preparations. Due to increasing demand, the natural occurrence of caffeine is complemented by synthetic production. This article describes the discovery, properties, and appearance of caffeine.