Many studies have been done to detect the effects of time out for individuals with mental retardation. But only a few studies have examined the effectiveness of time out as an intervention strategy for those with mental retardation who exhibit aggressive behavior. Currently, twelve empirical studies using time out on the treatment of aggressive behavior for students with mental retardation are reviewed. The review critically examined the effects of time out for such students. A percentage of nonoverlapping data (PND) method was used to analyze treatment effects in these studies. In addition, the analyses emphasized how research dealt with the reliable observation, internal validity, and ethical issues and durations of time out procedure. Finally, suggestions were made for future studies.