Since the early 1990s, primary immunodeficiency (ID) disorders have played a major role in the development of human gene therapy. Adenosine deaminase (ADA) deficiency was the first disease to be treated with a gene therapy approach in humans, and was also the first condition for which therapeutic gene transfer into the hematopoietic stem cell has been attempted in the clinical arena. A series of encouraging results obtained in chronic granulomatous disease (CGD) patients have followed these pioneer experiments and preceded the very recent and exciting reports of successful genetic correction procedures performed in patients affected with the X-linked form of severe combined immunodeficiency (XSCID). The technical progress made in the field of gene transfer in recent years is mostly responsible for these clinical advances, and will be critical for future development of gene therapy approaches for other forms of IDs.