Boston Teacher Residency (BTR) is an innovative practice-based preparation program in which candidates work alongside a mentor teacher for a year before becoming a teacher of record in the Boston Public Schools (BPS). The authors found that BTR graduates are more racially diverse than other BPS novices, more likely to teach math and science, and more likely to remain teaching in the district through Year 5. Initially, BTR graduates for whom value-added performance data are available are no more effective at raising student test scores than other novice teachers in English language arts and less effective in math. The effectiveness of BTR graduates in math improves rapidly over time, however, such that by their 4th and 5th years they outperform veteran teachers. Simulations of the program's overall effect through retention and effectiveness suggest that it is likely to improve student achievement in the district only modestly over the long run. (Contains 22 notes, 3 figures, and 7 tables.)