This chapter offers guidance for clinicians conducting clinical interviews to assess and diagnose major anxiety disorders, including specific phobias, panic disorder, agoraphobia, social anxiety disorder, and generalized anxiety disorder. The behavioral model offers a compelling conceptualization of each of the major anxiety disorders and offers a framework for organizing the diagnostic criteria for each disorder. A functional behavioral assessment approach identifies information that is needed for diagnosis and treatment planning. Familiarity with the functions of the client’s anxiety and avoidance behavior facilitates the process of gathering information. It also adds to the potential therapeutic value of the diagnostic interview by focusing attention on the functions of the client’s symptoms during the diagnostic process, potentially helping clients to feel understood and creating opportunities for the client to think differently about their problem. Interviewing protocols that have been empirically evaluated are reviewed. Other tools that can inform the diagnostic process are also discussed. The implications of reliance on self-report, the potential for distortion of information, and the activation of anxiety and avoidance during the interview are discussed. Consideration of attitudes toward the assessment process, and the impact of context on those attitudes, can help clinicians recognize and understand the variability in clients’ reports and clinical presentations.