This article examines a premise underlying teacher accountability policies, namely, that annual student learning gains can be attributed to individual teachers. After analyzing data collected in fourth- and fifth-grade reading and mathematics classes in 18 schools, the authors identify forms of instructional design that rely on multiple teachers. These designs--simultaneous, supplemental, and sequential--were pervasive, particularly in higher-poverty schools, raising policy-relevant questions about holding individual teachers responsible for student learning. The authors further argue that accountability systems that focus on individual teachers might adversely affect other promising school reform efforts, such as teacher collaboration and data-based decision making. (Contains 2 tables and 6 notes.)