The appearance of all animal phyla in a narrow geological stratum at the base of the Cambrian has been one of the greatest discoveries of all times, but has also been, and still is, one of the greatest unsolved problems of biology. What has to be explained is primarily the origin of all animal phyla in a geologically brief period of time, but there are other two closely related problems which are also waiting for an answer. One is the fact that the animal body plans have been strongly conserved after the Cambrian explosion, but not before. The other problem comes from embryology. The body plan is built during development and becomes visible at the phylotypic stage, the brief period in which the embryos of all the species of a phylum have the greatest degree of similarity. Before and after that stage, the embryos of different species become increasingly diverse, but in a very puzzling way. Before the phylotypic stage the differences are decreasing, whereas after that stage they are steadily increasing. The Cambrian explosion, in short, gives us three major problems: the origin of the animal phyla, the conservation of the body plan and the conservation of the phylotypic stage. In this chapter it is shown that if animals are described as idealised multicellular structures which are reconstructed from incomplete information, all three problems can be solved. Needless to say, it is a solution of an idealised Cambrian explosion obtained with idealised organisms, but the principles involved should not be dismissed lightly.