Quality teachers are the single greatest determinant of student achievement. Teacher education, ability, and experience account for more variation in student achievement than all other factors. Studies have found that 40 to 90 percent of the difference in student test scores can be attributed to teacher quality. Knowing the subject matter, understanding how students learn, and practicing effective teaching methods translate into greater student achievement. Therefore, it is vitally important that teachers be well prepared when they begin teaching and that they continue to improve their knowledge and skills throughout their careers.