It is argued that lateral masking is a composite of several processes. These processes include response competition, distribution of attention, perceptual grouping, and feature (contour) interaction. Three experiments were carried out in an attempt to isolate some of the components. In the first two experiments, it was shown that feature interaction dominates at close spacing but other processes dominate at wider spacing. The third experiment showed that at least part of the effect of perceptual grouping appears to be information provided about target location.