Past research has proposed various macrolevel theories of issue definition and agenda setting. However, we propose a microlevel theory of issue definition rooted in how individuals process information. We theorize that people process information about policy issues through a filter that emphasizes past assessments, ideology, background, social cues, and the continuing intrusion of new information. Most of these factors lead individual issue definitions toward stability. However, the introduction of an information signal of appropriate magnitude and character can produce punctuations in issue definition by individuals through time. Since the macrolevel definition of an issue is a type of aggregation of individual definitions, understanding how individuals define issues becomes a precursor to understanding issue definition at the system level. In evaluating the theory, we develop and evaluate a survey to study the issue definition process for individuals across multiple issues, and for global warming specifically. The survey also includes two embedded experiments to demonstrate the potential for punctuation in the issue-definition process for individuals and the system.