Every day, we interact with people synchronously, immediately understand what they are doing, and easily infer their mental state and the likely outcome of their actions from their kinematics. According to various motor simulation theories of perception, such efficient perceptual processing of others' actions cannot be achieved by visual analysis of the movements alone but requires a process of motor simulation-an unconscious, covert imitation of the observed movements. According to this hypothesis, individuals incapable of simulating observed movements in their motor system should have difficulty perceiving and interpreting observed actions. Contrary to this prediction, we found across eight sensitive experiments that individuals born with absent or severely shortened upper limbs (upper limb dysplasia), despite some variability, could perceive, anticipate, predict, comprehend, and memorize upper limb actions, which they cannot simulate, as efficiently as typically developed participants. We also found that, like the typically developed participants, the dysplasic participants systematically perceived the position of moving upper limbs slightly ahead of their real position but only when the anticipated position was not biomechanically awkward. Such anticipatory bias and its modulation by implicit knowledge of the body biomechanical constraints were previously considered as indexes of the crucial role of motor simulation in action perception. Our findings undermine this assumption and the theories that place the locus of action perception and comprehension in the motor system and invite a shift in the focus of future research to the question of how the visuo-perceptual system represents and processes observed body movements and actions.action perception | motor simulation | mirror neurons O ur social life rests in large part on our capacity to perceive and interpret others' behavior accurately to anticipate their upcoming actions and adjust our own behavior appropriately. The impairment of this ability in the context of autism, schizophrenia, various types of dementia, or following stroke or head injury has tragic consequences for both patients and society. However, how the human mind and brain supports these abilities remains a major challenge. In this study, we addressed a specific issue regarding the relationship between visual and motor processes in action perception and recognition: Does efficient perception and interpretation of action rely on unconscious, covert motor simulation of the observed movements?The traditional view on this issue is that efficient perception and interpretation of human movement does not rely on motor simulation but rather on computations occurring in the visuoperceptual system supported by basic perceptual processes and information extrapolated from perceptual learning (1-5). This view has been challenged by a series of motor simulation theories of perception which, despite their differences, all assume that efficient perceptual processing of others' actions cannot be achieved by ...