Amphibia and its major groups are defined according to principles of phylogenetic taxonomy, and the implications of the definitions for amphibian systematics are discussed. The results of phylogenetic analyses of Amphibia, Anura, Caudata, and Gymnophiona from morpho- logical and molecular studies are compared, based on papers published in the symposium "Am- phibian relationships: Phylogenetic analysis of morphology and molecules" at the 1990 meetings of the American Society of Zoologists in San Antonio, Texas. Several issues related to the use of morphological and molecular data sets are discussed briefly: quality and quantity of data, homology assessment, nonindependence of characters, sampling of taxa, and resolution of trees derived from different data sets.