Expanded Learning Time (ELT) is seen as one promising approach for improving academic achievement, particularly in the nation's chronically low-performing schools. Since 1995, Citizen Schools (CS) has developed and supported the implementation of its own ELT model. The CS ELT model uses an additional shift of educators and community volunteers to engage middle school students in hands-on apprenticeships, while simultaneously providing individualized supports to ensure academic and future success. Theory suggests that as a result of ELT, students may become more engaged in school because of additional enrichment and learning opportunities, develop better communication skills due to more time with teachers and peers, and be less likely to engage in disruptive behavior because of less idle time. Ultimately, student achievement may improve as a result of additional learning time. The authors are conducting a five-year evaluation of the CS ELT model that includes an implementation and outcomes component to assess the operation and effects of ELT programming on middle school students. The implementation component of the study incorporates surveys, interviews, and site visits to assess how schools integrate ELT into their school days, organizationally and instructionally. This paper focuses specifically on how to characterize implementation systematically and in such a way as to explain variation in student achievement results.