About a third of students entering high school do not graduate and face limited job prospects. The No Child Left Behind Act requires states to use graduation rates to measure how well students are educated. To assess the accuracy of states' rates and to review programs that may increase rates, GAO was asked to examine (1) the graduation rate definitions states use and how the Department of Education (Education) helped states meet legal requirements, (2) the factors that affect the accuracy of states' rates and Education's role in ensuring accurate data, and (3) interventions with the potential to increase graduation rates and how Education enhanced and disseminated knowledge of intervention research. As of July 2005, 12 states used a graduation rate definition--referred to as the cohort definition--that tracks students from when they enter high school to when they leave, and by school year 2007-08 a majority plan to use this definition. Thirty-two states used a definition based primarily on the number of dropouts over a 4-year period and graduates. The remaining states used other definitions. GAO recommends Education provide information to all states on ways to account for different types of students in graduation rate calculations, assess the reliability of state data used to calculate interim rates, and establish a timetable to implement the recommendation in GAO's 2002 report to evaluate research and also to disseminate such research. Education agreed with GAO's recommendations on accounting for different types of students and the need for research.