The animal world is immensely diverse, and our understanding of it has been greatly enhanced by molecular biology and the study of evolution and development ('evo-devo'). Looking at genes, and especially key families of genes such as the Homeobox genes which control the development of body plans, has led to radical changes in the classification of animals. In this Very Short Introduction Peter Holland presents a modern tour of the animal kingdom. Beginning with the definition of animals (not obvious in biological terms), he takes the reader through the revolutionary new high-level groupings of animals (phyla) based on evolutionary relationships and ancestry. This phylogenetic view is central to zoology today and the volume will be of great value to all students of the life sciences, as well as providing a concise summary for the interested general reader.