This volume concentrates on the origin of multicellular animals, Metazoa. Until now, no unequivocal phylogeny has been produced. Therefore, the questions remain: Did Metazoa evolve from the Protozoa only once, or several times? Is the origin of animals monophyletic or polyphyletic? Especially the relationships between the existing lower metazoan phyla, particularly the Porifera (sponges) are uncertain. Based on sequence data of genes typical for multicellularity it is demonstrated that all Metazoa, including Porifera, should be placed into the kingdom Animalia together with the Eumetazoa. Therefore it is most likely that all animals are of monophyletic origin.