Principles of Development is designed for undergraduates. The emphasis is on principles and key concepts. Central to the authors' approach is that development can best be understood by understanding how genes control cell behaviour. They have assumed that students have some basic familiarity with cell biology and genetics, but all key concepts, like the control of gene activity, are explained in the text. 'We have resisted the temptation to cover every aspect of development and have, instead, focused on those systems that best illuminate common principles. Indeed a theme that runs throughout the book is that universal principles govern the process of development' (Lewis Wolpert, Introduction) The focus of the text is on vertebrates and Drosophila, but not to the exclusion of other systems, such as the nematode and the sea urchin, where they best illustrate a concept. An important feature of the book is the inclusion of the development of plants, which is usually neglected in other textbooks, but has some unique and important features. Principles are presented as clearly as possible and numerous summaries are provided, both in words and in pictures. The illustrations in the book are a special feature, and have been carefully designed and chosen to illustrate both experiments and mechanisms.