When people perceive a response (or outcome), whether their own or another person's, they represent it as being about something, and this thing that the response is about is inferred to be the source of the response. The first section of this article discusses natural assumptions underlying the operation of this aboutness principle and describes the problems with its use, illustrated by such well-established cases of social-cognitive shortcomings as the correspondence bias, representativeness error, misattribution, accessibility bias, and saying-is-believing effect. The second section uses the aboutness principle to review lay psychological theories discussed in this special issue, including people's theories of causality, stability, and change in personal attributes, conditions for valid memories or judgments, group attribute clusters, and persuasive influences.