Across the globe, educational tests are being used at a rapidly increasing rate. More recently, educational tests are being used to inform educational policy and for holding educators accountable for student learning. One reason educational assessments are used for these important purposes is that they are considered to provide reliable and objective information regarding students' achievement. The fact that these tests are "standardized," meaning the content, test administration conditions, and scoring are uniform (consistent) across all test takers, supports this perception of objectivity. However, no test is perfectly suited for all students and so educational tests typically do the best they can for the majority of the population tested (Geisinger, 2000). For this reason, it is important to consider identifiable subgroups of the examinee population when considering the fairness and appropriateness of educational tests. In the United States, one important subgroup that presents particularly difficult challenges to valid measurement is English learners (ELs). In this article, we define ELs and the difficult challenges inherent in assessing their academic knowledge, skills, and abilities.