The aim of the present article is to review the effects of caffeine on human behavior. The main areas of behavior reviewed are mood, mental performance and sleep. Certain areas, although related to behavior, such as the underlying CNS mechanisms, are not reviewed in detail. This is because most of the research in this area has involved animal studies. These have provided plausible mechanisms for many of the effects observed in humans, but whether specific details (e.g. effects of dose) generalize across species is unknown. Other areas, such as the claims that caffeine is addictive, are not covered here but are discussed in a paper by Dews et al. in this issue. In all areas it is important to make a distinction between the effects of amounts of caffeine that are normally consumed from food and drinks, and the very different effects observed with excessive amounts or in very sensitive individuals. Unlike other areas of research (e.g. studies of health effects), most studies of the behavioral effects of caffeine have examined acute changes following a single dose. Less is known about effects of regular consumption, although there are now enough data on this topic to draw tentative conclusions. In addition to studying the effects of caffeine consumption, the research has also considered possible changes in behavior as a function of caffeine withdrawal. A critical appraisal of claims that caffeine withdrawal influences performance and mood is provided. It is important to note that in research reviewed in all subsequent sections, there is considerable variation in the methodology and measuring devices. This has the disadvantage that it is often difficult to compare different studies. However, persistence of effects across a range of methodologies enhances the validity of the reported observations.