The tumor necrosis factor (TNF) receptor superfamily comprises 10 different members of type I integral membrane glycoproteins with characteristic limited sequence homology for the extracellular cysteine-rich repeats. A parallel existing TNF ligand superfamily has been discovered by cloning of ligands for all of the TNF receptor superfamily members. These molecules are type II membrane glycoproteins, with the exception of LT-alpha, which is the only secreted protein of the family. TNF and CD95L also exist in biologically active shed soluble form. The TNF ligand superfamily presently contains nine different proteins. In addition, the NGFR p75 binds to a second family of proteins. These NGF-like dimeric soluble molecules are basic neurotrophic factors, and the five members are not related to the TNF superfamily ligands. The TNF-like ligands share some common biological activities, but other activities appear to be shared only by some ligands or are unique. The diverse biological effects mediated through the interaction of the members of the TNF receptor and ligand superfamilies have provided information on the regulation of cellular activation, including the involvement of T-cell-dependent immune responses as well as associations with human diseases.