Gene Therapy is expected to revolutionize the practice of medicine at the turn of the third Millennium. Therapeutic/prophylactic benefits should arise from both gene transfer and gene repair/inactivation protocols devised for patient's somatic cells. Gene expression cassettes, designed for the production of therapeutic proteins and non-coding RNA, are thus experimented together with emerging gene repair/inactivation techniques on a variety of inherited, acquired and infectious/parasitic diseases, including complex neuro-degenerative processes. This book presents a collection of chapters on the main aspects of Gene Therapy, some of which have already been treated in the past, and updates and further develops the current survey. It is aimed at understanding why Human Gene Therapy is likely to be a medical breakthrough, although definitive clinical success still needs time to accomplish. The contributions focus on both technical/biomedical concepts and on experimental/clinical data that appear to confer potential universality to Gene Therapy. This book does not claim to provide an exhaustive review of the pathologies, which are currently approached with Gene Therapy. The presentation of current and emerging approaches, together with problems and tentative issues aims to serve as a booster for the development of new applications in every field of medicine and will, therefore, be of value and interest to researchers in this exciting field.