Molecular Principles of Animal Development by Alfonso Martinez-Arias and Alison Stewart is the only textbook in the field that focuses on the molecules and mechanisms that underlie the developmental process. Offering a compellingly modern way of looking at developmental biology, it reflects recent literature and the dramatic advances and changes in the field over the last decade. It works from the molecular and cellular principles up to the organism level, rather than the traditional top-down emphasis on the complete organism. The authors demonstrate that an understanding of animal development requires an understanding of the universal mechanisms that emerge from molecular networks. Designed for advanced undergraduates and early graduate students, this well-organized book can be used to teach courses at different levels. It includes an introduction and three sections that increase in level of complexity while building on previous sections. It shows that development is not programmed into the genome but is an output of the opportunistic use of molecular networks and routines. Written with a rigorous but readable style, Molecular Principles of Animal Development is a stunningly useful addition to the field of developmental biology.