Book excerpt: Interrelationships of the Platyhelminthes elucidates the role of the flatworms in the animal kingdom. It brings together results from an international group of experts, spanning many disciplines, who give evidence for the phylogeny of flatworms and constituent major taxa. A combined approach, using traditional comparative techniques along with the modern techniques of molecular phylogeny, is utilized to show that the monophyly of the phylum is not fully established, and that the phylum may in fact consist of two groups: the acoels and their relatives, which are basal metazoans, and the Rhabditophora, which is a more derived group.