In adolescence research, the treatment of measurement reliability is often fragmented, and it is not always clear how different reliability coefficients are related. We show that generalizability theory (G-theory) is a comprehensive framework of measurement reliability, encompassing all other reliability methods (e.g., Pearson "r," coefficient alpha and "KR-20," intraclass correlation coefficients). As such, G-theory provides the flexibility and comprehensiveness not offered by conventional reliability methods. Within the G-theory framework, the similarities and differences of different reliability coefficients can be easily understood, and planning for optimal measurement protocols in adolescence research is feasible. Using hypothetical data, we show how different conventional reliability estimates are related to G-theory estimates and how G-theory estimates are obtained in research practice. Adolescence researchers may use G-theory as the general framework for understanding different reliability estimates.