In this chapter, we describe basic and expert competencies for the use of single subject designs. Basic competencies include repeated observation of behavioral phenomenon of interest, the participant serving as his or her own control, and utilizing measurement and visual analysis strategies that allow one to determine whether change occurs over time. Expert competencies include basic competencies plus repeated demonstration of experimental effect and altering only one variable at a time when introducing treatment. Through the use of numerous examples, we illustrate how single subject research designs lend themselves well to various aspects of research, including the stage of theory development (Level 1 research) and more formal and systematic tests of said theories (Level II and Level III research). We also describe how basic and expert competencies of single subject designs can be incorporated into clinical practice to evaluate effects of intervention. The reader will learn how single subject designs are unique approaches to scientific inquiry due to several features, including repeated observation of the dependent variable(s), replication of treatment effects, intrasubject and intersubject comparisons, visual analysis of individual participant data, and systematic manipulation of independent variable(s). This chapter will likely be useful for clinicians and researchers alike who are interested in answering questions about clinical phenomena in a manner that meets the requirements of the scientific method.