This fourth edition of Alan E. Kazdin’s classic text is, like its predecessors, intended to help students and professionals alike master a wide range of methodological approaches to examining clinical issues and phenomena. The goal is to help the reader design, conduct, recognize, and appreciate high quality research, and recognize the implications of crucial decisions about methodology and design. Articles cover a comprehensive array of topics, including experimental design; the principles, procedures, and practices that govern research; assessment of study constructs and their interrelationships; potential sources of artifact and bias; methods of data analysis and interpretation; ethical issues; and the nuts and bolts of writing research articles and getting published. With 26 new articles and significantly revised and expanded introductory material, this revamped edition features scholarly contributions that explicate core concepts, survey contemporary issues in research, and examine ethical responsibilities toward both research participants and science itself. New additions include articles on translational and qualitative research, advances in data collection methods such as Amazon’s Mechanical Turk service and obtaining client feedback in psychotherapy, advances in mathematical and statistical modeling including single-case interventions, and new chapters addressing questionable research practices and fraud.