What is human enhancement? This term has become so common that it hardly seems to need clarification. And yet this is precisely the issue we raise in this book: the definition, scope and limits of human enhancement are as vague as the term is salient. Books explicitly devoted to human enhancement have already been published, mainly by philosophers, but most of these books deal with its ethical and political aspects, such as: Is it good and desirable or bad and dangerous? What moral criteria can help us shape our judgements? A selection of such books are listed at the end of the introduction; many are referred to throughout this book. Less attention, however, is devoted to questions about what exactly is being studied and how it should be approached.