In this chapter, the authors will describe the four types of validity: construct validity, content validity, concurrent validity, and predictive validity. Depending on the test and the rationale or purpose for its administration, and understanding of the different types of validity can give teachers useful information about students’ test results. Moreover, if test results are going to be used to help make educational policy decisions, a knowledge of validity will give teachers and educational planners a more scientific basis for measuring student achievement. Therefore, to be more confident in the systematic and empirical interpretation of test results, teachers should understand the four types of validity.