Dylan William argues that if we are serious about raising student achievement, the only way to do this is by investing in the professional development of teachers. Not all investments are equally effective, and some, such as helping teachers make greater use of assessment for learning, are more effective than others, such as increasing teacher content knowledge. If we are to be successful, however, as well as being clear about what we want teachers to do differently, we have to understand why changing teachers' practice is so difficult.