Gene therapy consists in the introduction of genes in cells of the human organism. It allows the treatment of not only monogenic diseases (those in which one gene is altered, such as rare enzymatic diseases), but also of diseases in which there is an acquired gene alteration such as AIDS and cancer, and diseases in which several genes and the environment interact, such as diabetes and coronary artery disease. In the few years of development of this therapy several clinical trials have been approved, that, along with experimental research, are offering promising results. In this new field of modern medicine, specially in experiments related to genetic manipulation of germinal cells, ethical aspects have great importance.