As has been noted at various points in this volume, criminal investigation has long been a staple of popular culture. Indeed, it appears that modern television is all but obsessed with forensic science. From Cracker to Crime Scene Investigation, the schedules are now full of images of highly skilled experts and technicians ‘cracking’ crime through the appliance of science. Now, of course, the success of such dramas is in part that they play to our long-held desire for security and to our belief in the efficacy of scientific knowledge in making the world a better and safer place. One doesn’t need to be a signed-up member of the society of postmodern relativists, however, to recognize that there is as much wishful thinking in this as there is truthful representation. Of course, forensic sciences don’t lead straightforwardly to a massive hike in clear-up rates or necessarily to solutions to the most complex criminal investigations. And, yet, clearly they have huge potential and represent a significant advance. But how do we assess them? Separating the factual wheat from the fictional chaff is the primary purpose of the chapters in this part. What are forensic techniques, what do they involve and what are their promises and limitations?