Gene therapy is defined as genetic modification of cells to prevent/treat disease. It also provides extra copies of normally expressed gene. Vectors deliver deoxyribonucleic acid (DNA) material to target cell. Ideal vector is safe and effective. Most important viruses used in biological vectors are adenovirus and retrovirus. Earlier, vectors are used only for inherited single gene defects, but presently used for many acquired disorders. Clinical applications of vector are covered in this chapter.