Gene therapy considered to be a technology based on advancements in molecular biology originally conceived as an approach to treat autosomal recessive Mendelian disorders, now being applied to a broad range of acquired conditions such as cancers, infections and degenerative disorders. At the beginning of the nineties, much of this young field was based on a relatively simple concept: a deficient function can be replaced with an artificial gene. For twenty years now, gene therapists have been learning how wide the gap is between a concept and its practical reality. It will be of interest over the next few years to analyze whether clinical results will reach the expectations in this domain bearing in mind the lessons of gene therapy for cancer. The treatment of cancer, makes up almost 60 to 70% of the gene therapy trials worldwide, and includes some very elegant approaches.