Clinicians order specific laboratory tests and interpret laboratory data that will be useful in establishing a diagnosis of immunodeficiency, either inherited (primary) immunodeficiencies or acquired (secondary) immunodeficiencies. The clinical immunologic evaluation of patients for immunodeficiency proceeds in an orderly fashion, from screening tests to sophisticated tests. The medical history and physical examination of the patient frequently provide clues guiding the level of entry into this testing program. Specific tests have been designed to screen for the four basic mechanisms of host defense: Antibody, T cell, phagocyte, and complement. The clinical immunology laboratory is a powerful adjunct to the clinician in the initial evaluation of immunodeficiency disorders and in the design of more sophisticated testing for selected patients.