Teacher evaluation policies seek to improve student outcomes by increasing the effort and skill levels of current and future teachers. Current policy and most prior research treats teacher evaluation as balancing two aims: accountability and skill development. Proper teacher evaluation design has been understood as successfully weighting the accountability and professional growth dimensions of policy and practice. I develop a model of teacher effectiveness that incorporates improvement from evaluation and detail conditions which determine the effectiveness of teacher evaluation for growth and accountability at improving student outcomes. Drawing on empirical evidence from the personnel economics, economics of education and measurement literatures, I simulate the long-term effects of a set of teacher evaluation policies. I find that those that treat evaluation for accountability and evaluation for growth as substitutes outperform policies that treat them as complements. I conclude that optimal teacher evaluation policies would impose accountability on teachers performing below a defined level and above which teachers would be subject to no accountability pressure but would receive intensive instructional supports.