Gene therapy makes use of cloned DNA molecules as pharmaceutical agents aimed at the treatment of both inherited genetic disease as well as acquired disorders such as cancer and viral infections. A major hurdle to be overcome in this approach is the introduction of genes into cells. Both viral and non-viral vectors have been used as ‘gene ferries’ and the design of vectors with desired specificities is under way. Today, the field has reached a stage where one can safely say that the principle of gene therapy has been proven and has been shown to represent a realistic therapy for patients, but much development is still required, especially in the area in in vivo gene transfer.