The sixth edition of Introduction to Forensic and Criminal Psychology caused me to revise a lot of the material which had built up through previous editions. The main reason is that forensic and criminal psychology as an area of research is now substantial and well-established. Just what underlies my thinking when preparing a new edition? Most important is my intention to present forensic and criminal psychology as a vigorous, developing discipline. Forensic and criminal psychology is not simply a compendium of settled facts. Despite its still relatively short history, like other disciplines, it gradually develops, building on earlier achievements. It is important to capture this so that the reader can sense this progression. This is not entirely an orderly, even and predictable process. Ideas in some fields of research have changed markedly – even reversing – since the early editions of this book. These developments exemplify not a series of errors but the way in which disciplines advance. We can see this both in the short term and the long term. It should also be possible to spot the unevenness of this progress. In its simplest form, over the various editions once-hot topics have gradually received less attention and Cinderella topics have blossomed markedly. At its most mundane, this means that some chapters have been updated with lots of new material, while a very few have changed little. No doubt there will be a reversal of the fortunes of some of these in future years. Nevertheless, some traditional topics have been retained simply because they were important in the early days of the discipline. This is particularly the case with FBI-style offender profiling.