An increasing challenge to physicians is represented by a sizable population of patients who suffer from immunodeficiency diseases. These diseases may involve deficiencies or imbalances of the specific immunologic defenses represented by the T-cell-mediated immunities, B-cell-mediated or humoral immunities, major biologic amplification systems of either cell-mediated or humoral immunities, and the fundamental effector mechanisms. The consequent extraordinary heterogeneity of these diseases implies that curative, corrective, or even replacement and supportive therapy, to be most effective, must be based on fundamental understanding of the nature of each disease, its specific diagnosis and analysis of its pathogenesis. Although much progress has been made toward understanding these diseases over the past 25 years, analyses are often incomplete, sometimes controversial, and occasionally entirely lacking, rendering therapeutic efforts less than ideal. Nonetheless, dramatic advances in therapy have been forthcoming. With rapidly improving understanding of the basic deficiencies underlying the diseases and with improvement in the therapeutic armamentarium, one of the most rewarding and indeed dramatic branches of medicine has been that derived from continuing efforts to treat patients with primary immunodeficiency diseases. It will be our purpose in this chapter to outline this rapidly developing field from a focus in our own experience with large numbers of patients having all currently recognized forms of primary immunodeficiency. It seems likely, from data and new therapeutic agents already at hand and under clinical and laboratory investigation, that the therapy of primary immunodeficiency will continue to develop rapidly in the years ahead and will surely be an important guide to improved treatment of secondary immunodeficiencies as well.