Originally identified as a cell surface receptor that triggered the death of lymphocytes and tumor cells, it is now recognized that Fas (also known as CD95 or Apo-I) has distinct functions in the life and death of different cell types in the immune system. Fas signaling may also be involved in T cell costimulation and proliferation. Although Fas deficiency in humans and mice predisposes them towards systemic autoimmunity, Fas-FasL interactions can also facilitate organ-specific immunopathology. Proximal signaling by Fas and related receptors depends on subunit preassembly, which accounts for the dominant-negative effect of pathogenic receptor mutants and natural splice variants.