Tumor necrosis factors (TNFs) have been a focus of research for well over a decade now. The identification and recent molecular cloning of two different types of cell-surface TNF receptors will shed further light on the mode of action of these pleiotropic cytokines. In the present article, we summarize the data on the biochemistry and structure of the receptors and focus on the molecular cloning of the respective cDNAs. The nucleotide sequences of the receptor genes revealed that both TNF receptors belong to the still growing nerve growth factor receptor gene family. The function and origin of TNF inhibitory proteins as well as receptor-mediated signal transduction are discussed.