A major conclusion is that polygraph testing is a complex process that varies widely in application. While the polygraph instrument itself is essentially the same for all applications, the purpose of the examination, type of individual tested, examiner training, setting of the examination, and type of questions asked, among other factors, can differ substantially. The instrument itself cannot detect deception; therefore, the polygraph test requires the examiner to develop questions for each case, compare the physiological response (as measured by the instrument) to the different questions, and infer deception or truthfulness based on these comparisons. A second major conclusion is that no overall measure or single statistic of polygraph validity can be established based on available scientific evidence. The amount and quality of the evidence depends on the particular application. Therefore, conclusions about polygraph validity can only be made in the context of specific applications. The types of applications discussed are specific-issue criminal investigation, specific-issue screening investigation, and personnel security screening, including preemployment or preclearance, and aperiodic. Tabular data and 204 references are provided.