The authors conducted a quasi-experimental investigation of effects on achievement by grade-level teams focused on improving learning. For 2 years (Phase 1), principals-only training was provided. During the final 3 years (Phase 2), school-based training was provided for principals and teacher leaders on stabilizing team settings and using explicit protocols for grade-level meetings. Phase 1 produced no differences in achievement between experimental and comparable schools. During Phase 2, experimental group scores improved at a faster rate than at comparable schools and exhibited greater achievement growth over 3 years on state-mandated tests and an achievement index. Stable school-based settings, distributed leadership, and explicit protocols are key to effective teacher teams. The long-term sustainability of teacher teams depends on coherent and aligned district policies and practices. (Contains 6 tables and 2 figures.)