The reductionist programme in biology has yielded amazing insights into the basic processes of life. In particular, the explanation of many cellular processes at the molecular level has revolutionized our understanding of biology. However, given the vast increase in the amount of analytical information now being obtained, we have to ask how much we really need to know in order to 'understand' a biological process. The reductionist programme raises fundamental questions about levels of explanation. This book brings together a celebrated group of philosophers and scientists to discuss these problems. Some chapters deal with the enormously powerful techniques of molecular biology, and analyse precisely how molecular information has improved our understanding of biological processes. Others deal with specific physiological systems in relation to the appropriateness of attempts to provide reductionist explanations. The area of brain and behaviour is given particularly close attention because there are a number of differing views held by scientists about whether an analytical and reductionist approach is even desirable in this context. Separate chapters deal with ecological and evolutionary issues. Providing a comprehensive account of the application of reductionist ideas in biology, this book deals with controversial ideas, particularly in relation to reductionist explanations of behaviour, and presents both sides of the argument. The issues raised are relevant to virtually every area of biological research.