Calls to fire the lowest-performing teachers are widespread, but such actions are not likely to improve teacher quality. Instead, educators should focus on improving the education system that can prevent even good teachers from providing high-quality teaching. To improve the teaching environment, educators and policy makers need to better understand the challenge. They must view the barriers to quality in terms of "systems," rather than merely the attributes of individual teachers. That's because variations in teaching performance flow largely from variables that have little to do with the qualities of teachers themselves. Thus, improving the quality of the nation's teachers won't come simply from trimming away the weakest performers.