Although caffeine is the most widely used behaviorally active drug in the world, caffeine physical dependence has been only moderately well characterized in humans. Four double-blind experiments were conducted in independent groups of healthy participants to assess the conditions under which withdrawal symptoms occur upon cessation of low to moderate doses of caffeine. In experiment 1, there was no evidence that the range or magnitude of caffeine withdrawal symptoms differed when 300 mg of caffeine was consumed as a single dose in the morning versus 100 mg at three time points across the day. In experiment 2, both the range and severity of withdrawal increased as a function of caffeine maintenance dose (100, 300, and 600 mg/day), with even the lowest dose (100 mg) producing significant caffeine withdrawal. Experiment 3 showed that when individuals were maintained on 300 mg caffeine/day and tested with a range of lower doses (200, 100, 50, 25, and 0 mg/day), a substantial reduction in caffeine consumption (</=100 mg/day) was necessary for the manifestation of caffeine withdrawal. Experiment 4 manipulated duration of exposure to caffeine (1, 3, 7, or 14 days of 300 mg/day) and showed that caffeine withdrawal occurred after as little as 3 days of caffeine exposure, with a somewhat increased severity of withdrawal observed after 7 or 14 days of exposure. As a whole, this set of experiments provides the most complete parametric characterization of caffeine withdrawal to date and suggests that caffeine physical dependence can occur under more modest conditions (i.e., fewer doses per day, lower daily dose, shorter duration of exposure) than previously recognized.