Cells of the immune system have a large number of protein receptors on their surfaces, with a wide range of binding functions. They are, however, constructed from a limited set of protein structural units, which are recognisable at the sequence level. The 3D structure of many of these domains, or modules, is now known. These modular units and their structures are reviewed here. The ways in which they are assembled into multidomain receptor chains and oligomeric complexes of receptors are also discussed.