This paper aims to provide guidelines on developing English language proficiency (ELP) tests based on the experience from ELP assessments in the U.S. after the implementation of No Child Left Behind (NCLB). While there might be substantial differences between the content and purpose of ELP tests developed from country to country, there are, however, areas that experts in charge of ELP test development in other countries can benefit from. The NCLB legislation in the U.S. made the assessment of English language learners (ELL) students’ level of proficiency in English mandatory once a year and provided useful guidelines for developing ELP assessments. This mandate, along with its useful guidelines, helped improve the quality of ELP assessment significantly and led to the development of several batteries of ELP assessments either through consortia of states or by test publishers in the U.S. The newly developed assessments were based on states’ ELP standards. They incorporated the concept of academic language which is an essential requirement for ELL students’ performance in the academic content areas, and were tested in extensive pilot and field studies. Some implications were drawn from such improvements for ELL assessment and accountability not only in the U.S. but in other countries including Korea.