Increased understanding of the nature and variety of immunodeficiency states in man is rapidly accumulating both from studies of human patients and from experimental work on the immune response in animals. Progress is evident in the development of diagnostic tests for deficiencies in both humoral and cellular mechanisms of immunity, and in the introduction of new forms of therapy-for example, the grafting of lymphoid cells. Studies of immunodeficiency provide the most direct evidence concerning the nature of the immune response in man, and hence are of wide general interest. In this paper, current knowledge and concepts are summarized, a logical classification is presented, and recommendations are made for the investigation and treatment of these disorders.