The high school dropout problem has been called a national crisis. Educators, researchers, and policymakers continue to work to identify effective dropout prevention approaches. One important element of such prevention efforts is the identification of students at highest risk for dropping out and then the targeting of resources to keep them in school. An early warning system that uses "indicators based on readily accessible data" can predict, during students' first year in high school, whether the students are on the right path toward eventual graduation. Research is clear that ninth grade is a "make or break" year. More students fail ninth grade than any other grade in high school, and a disproportionate number of students who are held back in ninth grade subsequently drop out. Recent research in large urban school districts, including Chicago and Philadelphia, provides information about powerful indicators that can predict, by the end of the first year of high school, or even during the first semester, whether students will complete high school. This brief guide reviews this research and uses it as a basis for providing guidance to schools and districts about using data to address the dropout problem. This guide, intended for educators and policymakers at the school, district, and state levels, is designed to provide information about the following: (1) Factors that contribute to a student's dropping out; (2) Research on early warning indicators; (3) School-level early warning systems; (4) District-level early warning systems; and (5) States' roles in supporting the development and use of early warning systems. (Contains 3 tables and 3 endnotes.)