The application of crystallographic techniques to Fab, Fc, and light-chain components of immunoglobulins has provided a three-dimensional framework for interpreting chemical and biological observations like those summarized by Metzger in the next chapter (p. 119) (Poljak et al.,1974, 1976; Segal et al.,1974; Padian et al.,1974; Davies et al., 1975; Deisenhofer et al., 1976; Epp et al., 1974; Fehlhammer et al.,1975; Wang et al., 1975; Schiffer et al., 1973; Edmundson et al., 1975). These high-resolution analyses have been supplemented by low-resolution studies of intact IgG1 and IgG2 immunoglobulins (Sarma et al., 1971; Silverton et al.,1977; Colman et al., 1976; Edmundson et al.,1970; Ely et al.,1978a; Colman et al.,unpublished data). The structures of immunoglobulin components are well described in the above articles and they will not be discussed in detail in the present report. Instead, we shall concentrate on recent crystallographic work relating to the capacities of immunoglobulins to assume different conformations.