Complement activation unleashes powerful effector mechanisms against which host cells are protected by homologous restriction factors. These factors are glycolipid-anchored membrane proteins that either induce C3 convertase dissociation (for example decay-accelerating factor) or prevent the full development of the membrane attack complex (for example homologous restriction factor and CD59). In this article Peter Lachmann explores the biology and biochemistry of these important and intriguing molecules.