The apparent potential of caffeine to enhance human performance has been widely exploited in selling caffeinated products. Self-ratings of the effects of caffeine intake provide the most direct measure of mood change as it affects the caffeine consumer. Caffeine-related mood changes are not always perceived as positive effects. Studies that consider whether caffeine can induce the subjective experience of mood change in healthy volunteers indicate that, in general, small but robust increases in perceived alertness can be detected with subjective rating tools, such as visual analogue scales, in deprived and nondeprived consumers. Earlier human studies, however, suggest that caffeine can reverse the mood impairments induced by the gabaergic agonists lorazepam and diazepam. The chapter shows that recurrent themes emerge regarding factors that modulate the robustness of caffeine-related performance effects. It summarizes the age and fatigue effects in some detail in the mood and the memory section.