The result of a two-year study, the report examines the history of the influential, century-old Carnegie Unit and its impact on education reform in K-12 and higher education. The study finds that the Carnegie Unit remains the central organizing feature of the vast American education system, from elementary school to graduate school, and provides students with an important opportunity-to-learn standard. But at best, the Carnegie Unit is a crude proxy for student learning. The U.S. education system needs more informative measures of student performance. Achieving this goal would require the development of rigorous standards, assessments, and accountability systems -- difficult work, especially in the field of higher education, where educational aims are highly varied and faculty autonomy is deeply engrained.