A meta-analytic review compared prospective and retrospective judgments of duration, or duration judgment paradigm. Some theorists have concluded that the two paradigms involve similar cognitive processes, whereas others have found that they involve different processes. A review of 20 experiments revealed that prospective judgments are longer and less variable than are retrospective judgments. Several theoretically important variables moderate these effects, especially those concerned with information processing activities. Therefore, somewhat different cognitive processes subserve experienced and remembered duration. Attentional models are needed to explain prospective judgments, and memory-based models are needed to explain retrospective judgments. These findings clarify models of human duration judgment and suggest directions for future research. Evidence on duration judgments may also influence models of attention and memory.