Validity refers to the degree to which interpretations of test scores that are used for a particular purpose are supported by both theory and empirical evidence. In this article, we discuss validity as it pertains to educational and psychological testing. We also trace the history of validity theory and discuss how to validate the use of a test for a particular purpose. We draw heavily from the Standards for Educational and Psychological Testing, which are developed by the American Educational Research Association, the American Psychological Association, and National Council on Measurement in Education, to promote quality test development and validation activities.