An Inaugural Professorial Lecture 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 and more suitable for large-scale application than others. In particular, he argues, helping teachers make greater use of assessment for learning (AfL), 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, and this will require radical changes in the way we treat teachers’ professional development.