The immunoglobulins are the most complex system of proteins known. They have no peers with respect to the structural diversity which they exhibit or with respect to the sophisticated mechanisms which must regulate their synthesis. On the other hand, there are few proteins which have been as thoroughly studied and about which there exists such a large body of information regarding structure, synthesis, catabolism, and function. Several detailed reviews of immunoglobulin chemistry have recently appeared 1,2 and several are in press. 3,4 In this paper, I will try to outline present concepts in a more general way. I have tried to organize the material in a logical framework so that the various levels of organization of the immunoglobulin system are highlighted. The Structure of Immunoglobulins There are three aspects of immunoglobulin structure which must be understood: All immunoglobulins have a common fundamental structure. Superimposed on this common subunit