This volume includes descriptions of a number of amphibian groups commonly referred to as lepospondyls. Three groups, now considered as orders, have long been included among the lepospondyls: the microsaurs, the nectrideans, and the a#stopods. Two clearly distinct families, the Lysorophidae and Adelogyrinidae, have been associated with the microsaurs or treated as separate groups within the lepospondyls. They are here placed in separate orders. The lepospondyls encompass a wide variety of small animals that differentiated near the base of the Lower Carboniferous and persisted for at least 75 million years, to the end of the Lower Permian. Together they show more anatomical diversity than do all the living amphibian orders. Despite the many autapomorphies of the individual groups, recent phylogenetic analysis does support a common ancestry of the lepospondyl orders.