Raising student achievement has benefits not only for the individual but for society as well. In this lecture Dylan Wiliam argues that many efforts at reform have focused on things that are easy to change rather than those that would make the most difference. He shows that some investments in raising achievement are more cost-effective & more suitable for large-scale application than others. In particular he argues helping teachers make greater use of assessment for learning (Af L) would be the most effective way of raising student achievement. Professor Wiliam argues that if we are to be successful as well as being clear about what we want teachers to do differently we have to understand why changing teachers' practice is so difficult & this will require radical changes in the way we treat teachers' professional development.