Across Arizona and the United States, there is widespread interest in how to successfully educate the growing number of English language learner students in K-12 schools. Research suggests that a school may face challenges in effectively teaching its English language learner students--and closing the achievement gap with native English speakers--when concentrations of English language learner students are high, especially in middle and high schools; when there are many socioeconomically disadvantaged students; and when the school is located in an urban or rural (as opposed to suburban) area. Research also suggests that open enrollment programs may increase the concentrations of English language learner students and socioeconomically disadvantaged students in some schools. This technical brief analyzes 2007/08 student-level data to determine how the number and percentage of English language learner students vary by public school in Arizona and how the percentage of English language learner students varies by school level (primary, middle, and high school), percentage of students eligible to receive free or reduced-price lunch (an indicator of socioeconomic disadvantage), school type (traditional, alternative, and charter), and school location. The study shows that statewide in 2007/08, 168,199 (16 percent) of Arizona public school students were classified as English language learner students. Their distribution throughout the 1,878 Arizona public schools in this study ranged from 0 to 853 students (0-87 percent) in individual schools. Schools and counties faced different levels of accountability and challenge in helping these students reach required levels of academic achievement, based on the number of English language learner students. Forty-one percent of Arizona public schools (those with 19 or fewer English language learner students) were not required to submit disaggregated adequate yearly progress reports for English language learner students or to implement the state English Language Development pull-out program. But 45 percent of schools (those with 40 or more English language learner students) were required to do both. Concentrations of English language learner students varied within and across schools and counties and by school characteristics. Schools with greater than 50 percent English language learner students were more prevalent among primary schools than among middle and high schools, among traditional public schools than among alternative and charter schools, and among schools with more than 75 percent eligibility for free or reduced-price lunch than among schools with smaller percentages of eligible students. Schools with no English language learner students were more prevalent in high schools, charter schools, and schools with less than 75 percent eligibility for free or reduced-price lunch. Differences were also evident by county. Maricopa and Pima Counties, with the largest total student enrollment among counties, had schools with 0 percent English language learner students close to schools with greater than 50 percent English language learner students. Of Arizona's 15 counties, the 6 with low student enrollment (1,000-50,000) had schools with 0 percent English language learner students and no schools with greater than 50 percent English language learner students. (Contains 10 tables, 4 figures, 1 map, 1 box, and 9 notes.)