Introspection involves looking inward into conscious thoughts, feelings, motives, and intentions. Modern social psychological research has raised questions about the value and reliability of information gained via introspection. This chapter concerns people's heavy weighting of introspective information for making self-assessments. It also concerns a few principles associated with that weighting-that is, that it does not extend to how people treat others' introspections, that it can lead people to disregard information conveyed by their own (but not others') behavior, and that it is rooted not only in people's unique access to their introspections but also in the unique value they place on them. Over-valuing of personal introspections occurs in a variety of domains, including judgment and decision making, personal relationships, and stereotyping and prejudice. An understanding of it sheds light on theoretical concerns involving the actor-observer bias, self-enhancement, temporal distance effects, and the perception of free will. People's unique valuing of their introspections likely has deep roots, but this "introspection illusion" also causes problems. It can foster conflict, discrimination, lapses in ethics, and barriers to self-knowledge and social intimacy. Understanding its sources and effects may help alleviate some of those problems.