Behavior checklists are questionnaires that are used to quickly assess a person’s functioning by having the individual or people close to them (i.e., parents, teachers, significant others) rate how often specific behaviors are observed. These questionnaires are widely used in assessment, research, and treatment to identify strengths and problematic behaviors to aid with diagnosis and/or identifying and tracking treatment goals. There are many different behavior checklists that are used to assess a wide range of areas; this entry reviews the development and format of behavior checklists, common areas that they assess (with examples of widely used checklists), and the advantages and disadvantages in using behavior checklists.