The main goal of our research was to investigate attention to dynamic visual stimuli. Observers were instructed to attentively track a variable number of moving visual objects for a sustained period of several minutes. At random intervals, a target stimulus of brief duration was presented inside one of these objects and observers were required to identify it. The number of relevant objects to be tracked and the grouping strength among objects that had to be attended, operationalized as the size of relative area among those objects, were manipulated. The results indicated that only a single object could be attentively tracked, unless the objects formed a perceptual group. In this case, more than one dynamic object could be attended, but effectiveness of attention would be mediated by perception of the spatial relations among them.