One of the mandates of the No Child Left Behind Act is that states show adequate yearly progress in their English language learners' (ELLs) acquisition of English language proficiency. States are required to assess ELLs' English language proficiency annually in four language domains (listening, reading, writing, and speaking) to measure their progress; they are also required to report on a composite comprehension measure. Often the clearest way to effectively monitor students' progress is to measure assessment results across grades on the same scale. In measurement terms, scores from tests across all grade levels can be put on the same scale using vertical scaling. In addition, to help stakeholders understand and interpret the results, these scale scores are often interpreted in terms of proficiency levels. In this article, we use the vertical scaling of WIDA ACCESS for ELLs[R], a large-scale K-12 Academic English Language Proficiency assessment to illustrate measurement and practical issues involved in this technique. We first give background on the need for vertical scaling. We then assess the literature on vertical scaling and describe the procedures used for WIDA ACCESS for ELLs[R] to vertically scale test scores and interpret the results in terms of the WIDA ACCESS for ELLs[R] Proficiency Scale. Next we review several studies that have been conducted to gauge the effectiveness of that scaling. We end the paper with a discussion of the broad issues that arise from vertical scaling. (Contains 2 notes, 8 tables, and 3 figures.)