Objects compete for representation in our limited capacity visual system. We examined how this competition is influenced by top-down knowledge using event-related potentials. Competition was manipulated by presenting visual search arrays in which the target or distractor was the only color singleton compared to displays in which both singletons were presented. Experiments 1 and 2 manipulated whether the observer knew the color of the target in advance. Experiment 3 ruled out low-level sensory explanations. Results show that, under conditions of competition, the distractor does not elicit an N2pc when the target color is known. However, the N2pc elicited by the target is reduced in the presence of a distractor. These findings suggest that top-down knowledge can prevent the capture of attention by distracting information, but this prior knowledge does not eliminate the competitive influence of the distractor on the target.