Purpose The purpose of this chapter is to help ensure that programs will gather and document appropriate evidence to support the intended inferences from reported test results and actions based on those inferences. Validity is one of the most important attributes of test quality. Programs should provide evidence to show that each test is capable of meeting its intended purposes. Validity is a unified concept, yet many different types of evidence may contribute to the demonstration of validity. Validity is not based solely on any single study or type of evidence. The type of evidence on which reliance is placed will vary with the purpose of the test. The level of evidence required may vary with the potential consequences of the decisions made on the basis of the test’s results. The validity evidence should be presented in a coherent validity argument supporting the inferences and actions made on the basis of the scores. Responsibility for validity is shared by ETS, by its clients, and by the people who use the scores or other test results. In some instances, a client may refuse to supply data that is necessary for certain validity studies. ETS cannot force a client to provide data that it controls, but ETS may wish to consider whether or not to continue to provide services to the client if ETS is unable to produce evidence of validity at least to the extent required by the following standards. Users are responsible for evaluating the validity of scores or other test results used for purposes other than those specifically stated by ETS, or when local validation is required. Because validity is such an inclusive concept, readers of this chapter should also see, in particular, the chapters on Fairness, Test Design and Development, Reliability, Scoring, and Test Use.