This paper examines the validity of measures of teacher effectiveness from the Tennessee Value Added Assessment System (TVAAS). It considers the following claims regarding teacher effects: (1) they capture teachers' unique contributions to student learning adequately; (2) they reflect adequate standards of excellence for comparing teachers; (3) they provide useful diagnostic information to guide instructional practice; and (4) student test scores capture desired outcomes of teaching adequately. Previous analyses of the TVAAS model and simulation studies highlight potential weaknesses in the system and identify gaps in the current record of empirical evidence bearing on its validity. (Contains 2 figures and 13 references.) (Author/SLD)