The single-case research design has become a paradoxical methodology in the applied sciences. While various experimental designs have been in place for over 50 years, there has not been wide acceptance of single-case methodology outside clinical and school psychology, or the field of special education. These methods were developed in the U.S.A., and their acceptance internationally has been even slower. The development and popularity of applied behavior analysis for persons with intellectual disabilities and the tremendously popular field of autism, and single-case designs have been very closely inter-related and have helped spread the use of single-case designs. The purpose of this paper is to review the history, types of designs, and their maturation in the applied fields of mental health and special education. (Contains 7 figures.)