"Single-case intervention research has a rich tradition of providing evidence about the efficacy of interventions applied both to solving a diverse range of human problems and to enriching the knowledge base established in many fields of science (Kratochwill, 1978; Kratochwill & Levin, 1992, 2010). In the social sciences the randomized controlled trial (RCT) experiment has, in recent years, dominated to the point that it has been acknowledged as the "gold standard" for the evidence-based practice movement (Shavelson & Towne, 2002). Single-case intervention designs have also been featured as an important methodology in this movement in that major task forces of the American Psychological Association (APA) recommend single-case designs alongside RCTs in their evidence standards. For example, the APA Divisions 12/53 (Clinical and Clinical Child, respectively) Task Force on Evidence-Based Interventions (Weisz & Hawley, 2000) and the APA Division 16 (School Psychology) Task Force on Evidence Based Interventions (Kratochwill & Stoiber, 2002) developed research-coding criteria for reviewing single-case intervention research. In education, the National Reading Panel (2000) included single-case designs in their review of reading interventions; and more recently the Institute of Education Sciences (IES) developed standards for review of single-case intervention designs to be incorporated by the What Works Clearinghouse (WWC)--see Kratochwill et al. (2010, 2013). Other professional groups and scholars have also featured appraisal criteria for review of single-case design research (J. D. Smith, 2012; Wendt & Miller, 2012). In short, single-case intervention designs are currently playing a major role in developing the research knowledge base in diverse scientific fields. The present volume endeavors to elaborate on and expand this role"--Introduction