It is increasingly common in English‐speaking countries that K‐12 school populations include students who learn English as a second language (ESL). This chapter focuses on the assessment of English language proficiency (ELP) of English language learner (ELL) students in K‐12 schools. It highlights the use of ELP assessments to exit students from ELL designation or ESL programs and move them into English‐only mainstream classrooms. The chapter presents a brief overview of large‐scale ELP assessment practice for ELL students in US K‐12 schools. Then it discusses issues in ELP assessment constructs, issues in developing and using ELP assessments for an exit decision, and validation considerations including technical qualities and consequences.