Primary immunodeficiencies are rare, but important for 3 reasons. First, a high index of suspicion and prompt diagnosis can lead to lifesaving treatment or significant improvement in quality of life. Second, appreciation of the genetic nature of a host defense defect makes possible family counseling and carrier and prenatal diagnosis. Finally, the large and growing list of human genetic defects in immune pathways provides an important tool for understanding human immunoregulation. Many inherited immunodeficiency diseases have had their genetic cause proven with the discovery of their disease genes within the past 5 years. These diseases provide a framework into which additional diseases and disease gene discoveries can be added as the rapid progress in molecular immunology and genetics continues.