English language learners (ELLs) are the fastest growing subgroup in American schools. These students, by a provision in the reauthorization of the Elementary and Secondary Education Act, are to be supported in their quest for language proficiency through the creation of systems that more effectively measure ELLs' progress across years. In the past, ELLs' progress has been based on students' prior scores measuring the same construct. To disentangle effectiveness from achievement, the reporting has generally targeted mean-group activity. In contrast, student growth percentiles (SGPs) provide a comparison of students' growth with others who have the same achievement score history. By examining the construct measured by an English language proficiency test as manifested in student scores in Speaking, Listening, Reading and Writing, this article outlines the use of SGPs in providing information on how much each student needs to grow, which will allow educators to more effectively apply differential formative instructional strategies.