English Language Learner (ELL) programs are intended to provide services to students with limited proficiency in English. These students are identified by their performance on language proficiency exams. ELL programs have been a requirement under provisions of No Child Left Behind. The title is often used interchangeably with ESL, or English as a Second Language, but many states use the ESL designation to describe the actual services that are provided to ELL students. According to statistics provided by the National Council of Teachers of English, ELLs are the "fastest growing" segment of the school-age population in the United States. They represent just over 10% of that population. A significant achievement gap exists between ELLs and the rest of the population, one which, given their numbers, represent a major challenge to education in this country.