An attempt has been made to provide the reader with an overview of the immune system and its clinical and laboratory evaluation. The assessment of this activity is aided by the realization that certain defects predispose the host to certain types of infection; historically then, one has a clue to the nature of the defect. Simple laboratory tests are available at most hospital and commercial laboratories, and many immune defects can be delineated through these techniques. Additionally, more sophisticated laboratory techniques are obtainable at specialized centers, although many of these studies are of research importance only. It is probable that over the next several years, newer technologies will permit an even more precise evaluation of the immune network and its defects.