Gene therapy has undergone a remarkable development in the last 20 years. Particularly important advances have been made in the improvement of gene transfer and expression technology, with current efforts focusing on the design of safer and long-term gene expression vectors as well as systems possessing cell-type specificity for transgene delivery and regulat‐ ability of its expression by small molecules. The foreign genetic material can be administered in vivo, ex vivo or in vitro depending on the nature of a disease. A successful gene therapy system must perform several functions. In all cases, the therapeutic gene must first be delivered across the cell membrane, which is a significant barrier. Once delivered inside the cell, the therapeutic gene may exist episomally or be integrated into the host genome depending on the nature of the gene transfer vector. Moreover, an important issue is the replication and segregation of the therapeutic gene during cell division in order to maintain long-lasting gene expression. These specifications will be discussed for each gene delivery system along the chapter.