The general meaning of gene therapy is to correct defective genes that are responsible for disease development. The most common form of gene therapy involves the insertion, alteration or removal of genes within an individual's cells and biological tissues. Many of gene transfer vectors are modified viruses. The ability for the delivery of therapeutic genes made them desirable for engineering virus vector systems. Recently, the viral vectors in laboratory and clinical use have been based on RNA and DNA viruses processing very different genomic structures and host ranges. Various viral vectors have been developed and optimized, such as retrovirus, adenovirus, lentivirus and adeno-associated virus. This book provides broad.