Single‐case design research has gained recognition as a credible scientific methodology across a variety of fields of study. Uniquely suited to support research on topics related to rare disorders or in instances where access to large sample sizes is unfeasible, single‐case designs have served to advance evidence‐based practices and assessment technologies. In applied settings single‐case designs provide opportunities for practitioners and researchers to be involved in advancing both the practice and research enterprise. Frequently used to examine issues in the areas of special education and applied behavior analysis, single‐case research has the potential to widely advance the field of clinical psychology. The What Works Clearinghouse of the Institute for Education Sciences has recently developed design standards and evidence criteria that can more specifically guide single‐case design research. In this chapter we review the major classes of single‐case design research (i.e., ABAB , alternating treatment design, multiple baseline design) and provide specific examples of the application of this quantitative methodology. Future directions and implications are discussed.