Severe congenital immunodeficiency diseases occur in approximately 1:10,000 newborns, in Western Europe and North America. They are characterised by recurrent infections, mostly caused by opportunistic micro-organisms, and autoimmune phenomena. In many patients the immune deficiency occurs as part of a syndrome. An immunological dysfunction may be caused by total absence, strong reduction or dysfunction of one or more cellular elements or of cell-associated or cellular secretion products. It is convenient to divide the immune system into a cellular compartment (with specific T and B cells, and non-specific natural killer cells and myelomonocytes), a compartment for cellular interactions (adhesion, costimulation and communication through cytokines) and an aspecific opsonization compartment. In cases of deficient humoral immunity treatment options are antimicrobial drugs and substitution therapy with immunoglobulins, in cases of phagocytic dysfunction the application of intracellularly active antibiotics and in cases of disturbances of cellular immunity allogeneic transplantation of haematopoietic stem cells and in the future perhaps the repair of the genetic defect by somatic gene therapy.