In this chapter, we describe how a simple attentional mechanism can account for a wide variety of phenomena in social perception. According to Attention Theory (Kruschke, 1996, 2003), people preferentially attend to differentiating information in order to maximize category learning. When learning multiple social categories, people attend to all features that characterize the first-learned category but shift their attention to features that uniquely distinguish a later-learned category from the first. As a result, they form a stronger impression of the later-learned social category. First, we review research on attentional processes in stereotype formation and group categorization. We show how Attention Theory can account for both category accentuation and illusory correlation in the formation of majority and minority group stereotypes. We then explain how attention shifting influences face perception and racial categorization. Second, we describe attentional processes as they relate to context-based impression formation and the influence of individual- and group-based expectancies on context-based impressions. Last, we discuss implications for impression change.