A complex visual scene will typically contain many different objects, few of which are currently relevant to behavior. Thus, attentional mechanisms are needed to select the relevant objects from the scene and to reject the irrelevant ones. Brain imaging studies in humans as well as neurophysiological studies in monkeys have identified some of the neural mechanisms of attentional selection within the ventral, “object recognition”, stream of the cortex. The results support a Biased Competition model of attention, according to which multiple stimuli in the visual field activate their corresponding neural representations throughout the cortical areas of the ventral stream. These representations engage in mutually suppressive interactions, which are strongest for stimuli occupying the same receptive field. The suppressive interactions are then biased in favor of one of the competing populations by “top-down” signals specifying the properties of the relevant stimulus in a given behavioral context. This top-down bias may originate in parietal and prefrontal cortex, and it is expressed in visual cortex at least in part through an increase in high-frequency (gamma) synchronization of neurons carrying critical information about the location or features of the behaviorally relevant stimulus. Conversely, low-frequency (beta) synchronization of neural activity may be relevant for suppressing distracters. High and low-frequency synchronization appears to be differentially present in superficial versus deep layers of the cortex, respectively, in visual areas V1 through V4, suggesting that they play different roles in feedforward and feedback connections in the cortex.