This article argues that the validity of standardized achievement test-score interpretation and use is problematic; consequently, confidence and trust in such test scores may often be unwarranted. The problem is particularly severe in high-stakes situations. This essay provides a context for understanding standardized achievement testing, then presents and discusses threats to validity, many of which are currently unaddressed. The public and several constituencies support standardized achievement testing. Many educators, however, especially educators in testing, have argued consistently that test-score interpretations and uses are inadequately validated. Standardized achievement test scores provide one valid source of information about student learning if they corroborate other information about student learning. Unfortunately, so many factors undermine the validity of test scores that we should be very careful in the way we interpret and use them.