The problems of resolving phylogenetic relationships change with increasing evolutionary distance: very widely separated organisms, which share few phylogenetically helpful morphological features pose the greatest difficulties. Thus, the resolution of relationships between animal phyla and classes present some of the most challenging problems in systematic zoology and paleontology. Traditionally, relationships have been inferred on the basis of comparative anatomy, embryology, and the paleontological record. However, phyla are established on the basis of the unique features of their body plans, and have few shared derived characters to unite them. Even when similar features are available, homology can still be uncertain. The fossil record has been largely unavailable to provide the needed continuity of structure to establish such homologies between phyla and most classes. Generally, the best data for connecting major groups have come from detailed embryological data. But, even there, many ambiguous situations and conflicts of interpretation remain, and these problems are pervasive over a wide range of animal groups, both with and without extensive fossil records.