Single subject design (SSD) research is a quantitative approach used to investigate basic and applied research questions. It has been used for decades to examine issues of social importance such as those related to general and special education strategies, therapeutic approaches in mental health, community health practices, safety, and business practices, to name a few. The purpose of this paper is to introduce the reader to the structure and function of SSD research, concepts of internal and external validity related to this methodology, and data analytic tactics used to evaluate SSD research. Additionally, several designs are described along with examples from the applied literature to illustrate the approach.