Internal validity is a scientific property that plays a critical role in the design, application, interpretation, and dissemination of experimental research. The strength of a study’s internal validity is based on the extent to which researchers have demonstrated experimental control, controlled for confounding variables, and minimized systematic error. Strong internal validity allows researchers to analyze data and report the results of a study with confidence. The value of research and its impact on professional practice, policies, and society as a whole is unquestionable. Therefore, scholars/researchers must demonstrate a thorough understanding of threats to internal validity, and how these threats apply to specific research methodologies. This paper examines the definition of internal validity as it relates to group design and single-case design research. Furthermore, specific threats to internal validity are outlined and methods/experimental designs employed by quantitative researchers to minimize those threats are discussed.