We focus on how standardized testing in American education has reflected, reproduced, and transformed social inequalities. We begin by describing inequalities in test score distributions by race/ethnicity, social origins, and gender over time. We then define learning, cognitive ability, and opportunity to learn, each of which influences the results of standardized tests. Next, we offer a brief history of standardized testing's role in American education. We then discuss the relationship between social stratification and measurement issues that arise in the context of standardized testing and the contemporary uses and misuses of standardized testing for diagnostic purposes, accountability, and gatekeeping. We conclude by reflecting on the past, present, and future role of testing in social stratification.