Primary immunodeficiency states result from developmental or functional failure within one or more of the immune compartments. While viral infections are rarely a serious consequence of complementor of phagocyte deficiencies, they are frequently seen with T or B cell deficiencies and may be associated with unusual presentation or pattern of disease. Here primary immunodeficiency is reviewed according to the WHO classification with consideration to the major effector system involved. Viral infections are considered in more detail in relation to those conditions in which they contribute significantly to infectious complications: antibody deficiencies, combined immunodeficiency and immunodeficiency associated with other major defects.