Dogs of different breeds can range remarkably in size, shape, and behavior, and yet they all carry essentially the same genome, making them a particularly fascinating model for genome plasticity. The recent release of the complete sequence of the dog genome provides an exciting new context in which to consider such variation. Twentyfive chapters written by experts in the field include various aspects of morphological and behavioral variation in dogs, their origins and domestication, and their unique value as a model system for many common but complex human diseases such as diabetes and cancer.