The authors review studies of mentally simulated movements. In automatic or cyclical movements, actual and motor imagery (MI) durations are similar. When athletes simulate only dynamic phases of movement or perform MI just before competing, however, environmental and time constraints lead to an underestimation of actual duration. Conversely, complex attention-demanding movements take longer to image. Finally, participants can modify the speed of MI voluntarily when they receive specific instructions. To complete the available data, the authors compared imagined and actual durations in tennis and gymnastics. Results showed systematic and disproportionate overestimation of actual duration. The authors found a relationship between complex motor skills and MI duration. They discuss the factors leading to over- and underestimation and the hypotheses that could be tested.