The advances of molecular biological techniques of the last decade have made possible identification of a rapidly increasing number of congenital (or primary) immune deficiencies at the genetic level. As soon as an immune deficiency is becoming a likely conclusion from a series of diagnostic laboratory tests, these recent developments in molecular techniques allow us in principle to make a conclusive diagnosis on the basis of a well-defined genetic defect. The hereditary immune deficiencies can be divided in 4 groups according to the functional immune aberration: antigen recognition, communication between immune cells, adhesion and directional motility, and killing as an effector mechanism.