Significant progress has been made in understanding the pathogenesis and molecular basis of the primary immunodeficiency diseases. Because genetically corrected cells of patients with these disorders may have a selective survival advantage, the primary immunodeficiency diseases have been a principal focus of initial clinical efforts at gene therapy. The technical challenges that must be surmounted for successful gene therapy and the approaches currently under investigation are described. The status of initial clinical trials of gene therapy for the primary immunodeficiency disorders is summarized.