Objective This review is intended to provide an outline for the evaluation of patients suspected of having immunodeficiency, a problem that is frequently encountered in clinical practice. Data Sources Information was obtained through a MEDLINE literature search as well as from standard textbooks in immunology. Also included is information that reflects the authors' clinical experience in the field. Results In general clinical practice, many physicians feel inadequate to evaluate patients with suspected immune deficiencies. They also think that the process of evaluation is time-consuming, which results in misdiagnosis of a substantial percentage of such disorders. Hence, the prevalence of immunodeficiency disorders is much higher than generally thought. At present, there are >80 unique primary immunodeficiency conditions and >50 syndromes that are associated with various immunologic defects. The prevalence of secondary immunodeficiency has also been increasing because of the tragic epidemic of HIV infection, more usage of immunosuppressive medications and bone marrow stem cell transplantation, and the severe degree of malnutrition in underdeveloped countries. It is necessary for clinicians, particularly the specialists in allergy and immunology, to be able to evaluate the status of the immune system. Conclusions Very valuable information can be gathered from the medical history and physical examination that may exclude or increase the suspicion of immunologic defect. Laboratory tests can then be appropriately selected to define the specific defect. Once the diagnosis has been settled, proper medical management can be instituted with subsequent improvement in morbidity and mortality of such disorders.