The main aim of the present study was to explore, by means of high-density EEG, the intensity and the temporal pattern of event-related sensory-motor alpha desynchronization (ERD) during the observation of different types of hand motor acts and gestures. In particular, we aimed to investigate whether the sensory-motor ERD would show a specific modulation during the observation of hand behaviors differing for goal-relatedness (hand grasping of an object and meaningless hand movements) and social relevance (communicative hand gestures and grasping within a social context). Time course analysis of alpha suppression showed that all types of hand behaviors were effective in triggering sensory-motor alpha ERD, but to a different degree depending on the category of observed hand motor acts and gestures. Meaningless gestures and hand grasping were the most effective stimuli, resulting in the strongest ERD. The observation of social hand behaviors such as social grasping and communicative gestures, triggered a more dynamic time course of ERD compared to that driven by the observation of simple grasping and meaningless gestures. These findings indicate that the observation of hand motor acts and gestures evoke the activation of a motor resonance mechanism that differs on the basis of the goal-relatedness and the social relevance of the observed hand behavior.