Behavior assessment describes the process of quantifying an operant dimension of student behavior, such that decisions can be made about the need for or effect of interventions. Assessment of behavior is essential to the successful implementation of intervention strategies, with failure of interventions often being attributed to lack of adequate assessment. The chapter describes the four primary purposes of behavioral assessment: universal screening, pre-intervention assessment, progress monitoring, and summative assessment. Next, the chapter describes various methods for assessment of behavior that may be utilized in school settings. Information is provided regarding direct behavior assessment and systematic direct observation, as well as systems for collecting such data on individual students and groups of students. Indirect behavior assessment through interviews, rating scales, office discipline referrals, and direct behavior ratings are also discussed. Finally, issues related to applied use of the described assessment tools are described.