We tested here the hypothesis that observing others' actions can facilitate basic aspects of motor performance, such as force production, even if subjects are not required to immediately reproduce the observed actions and if they are not aware that observation can form the basis for procedural training. To this end, we compared in healthy volunteers the effects of repeated actual execution (MOV) or observation (OBS) of a simple intransitive movement (abduction of the right index and middle fingers). In a first experiment, we found that both actual and observational training significantly increased the finger abduction force of both hands. In the MOV group, force increases over pre-training values were significantly higher in the trained than in the untrained hand (50% versus 33%), whereas they were similar for the two hands in the OBS group (32% versus 30%). No force change was found in the control, untrained group. In a second experiment, we found that both training conditions significantly increased the isometric force exerted during right index finger abduction, whereas no post-training change in isometric force was found during abduction of the right little finger. Actual performance, imagination and, to a lower extent, observation of fingers movement enhanced the excitability of the corticospinal system targeting the first dorsal interosseus muscle, as tested by transcranial magnetic stimulation; pre- and post-training effects were of similar magnitude. These results show a powerful, specific role of action observation in motor training, likely exerted through premotor areas, which may prove useful in physiological and rehabilitative conditions.