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.