What are plant growth regulators? In the title, and throughout the text, we have adopted this expression to describe a population of endogenous molecules and synthetic compounds of similar structure that are be­ lieved to play important roles in the regulation of plant differentiation and development. For many years, plant scientists have endeavoured to understand the nature and action of plant growth regulators and, as a result, an awesome quantity of written material now exists describing these chemicals and their effects. In this book we have aimed to distil this wealth of information into a more digestible form, and in particular we have focused our attention on a critical appraisal of the literature. The past few years have witnessed a change of emphasis in plant growth regulator research, which has been fuelled by powerful new techniques in molecular and cell biology. Today we can do more than just apply a plant growth regulator and quantify its effects; we have reached an exciting crossroads where plant scientists, molecular bio­ logists and chemists can pool their expertise and apply it to the out­ standing problems in this area. The combination of these three disciplines within the book is clear evidence of this. In keeping with a volume of this size, we have assumed that the reader has a sound knowledge of plant physiology and biochemistry. However, wherever possible, we have highlighted useful reviews which provide background information, along with recent publications that have contributed significantly to the literature.