The tumor necrosis factor (TNF) receptor family contains death receptors, which have a cytoplasmic death domain and can induce apoptosis, as well as receptors with no apparent homology in the cytoplasmic tail. This second group of receptors binds TNF receptor–associated factors (TRAFs), which are implicated in gene regulation and anti–apoptotic signaling. A bewildering variety of TNF receptor family members and their ligands are expressed on cells of the immune system. Based on the pattern of expression of receptors and ligands and based on the phenotype of available knock out mice, we summarize at which stages in lymphoid development and/or the peripheral immune response the various TNF receptor family members may play a role.