Efforts in gene therapy have grown dramatically in recent years. Basic research as well as clinical activity have made exciting progress and are beg- ning to offer renewed hope that gene therapy may be able to deliver novel approaches for the treatment of inherited as well as such acquired diseases as cardiovascular disease and cancer. With the sequencing of the human genome complete, we now have a comprehensive catalog of genes that further expands the potential role of gene therapy into such new fields as tissue engineering. Central to gene therapy is the process of gene transfer; thus, advances in the technology of gene transfer are at the heart of this field’s progress. Numerous technologies, based on a variety of methods (e.g., viral-mediated, physical/ chemical), have been developed to achieve gene transfer. Some of the earliest methods, such as recombinant retroviruses, are still widely used, have undergone significant improvements, and have given rise to new vectors based on lentiviruses.