This report describes the results of a project to develop a prototype computer-based system for Advanced Placement Computer Science (APCS) students. The APCS Practice System serves two purposes. First, it allows students to practice their programming skills and get immediate, diagnostic feedback about their productions. Second, it is a research tool for helping the College Board and ETS explore the integration of assessment with instruction and the introduction of new forms of assessment into large-scale testing programs. Pilot test results, current activities, and suggestions for future research and development are also described.