Gene therapy is a form of molecular medicine based on the addition of a corrected copy of a gene to the somatic cells of an individual in order to cure or to alleviate the underlying disease. This approach provides new targeted therapies for an increasing number of acquired or inherited diseases. Since its very first beginning in the 90's essentially for cancer treatment, this therapeutical approach has met unpredicted obstacles as well as true success. Recently, significant clinical benefits have been reported for several inherited diseases of the hematopoietic system but also of the retina and some cancers, widening its application provided that severe adverse events can be efficiently and stably prevented.