Erectile dysfunction (ED) is defined as the consistent inability to obtain or maintain an erection for satisfactory sexual intercourse. In the last decade, significant advancements in the pathophysiology of male ED have occurred. Basic science research on erectile physiology has been devoted to investigating the pathogenesis of ED and has led to the conclusion that ED is predominately a disease of vascular origin. Nitric oxide (NO) is the principal mediator of penile erection. NO is synthesized by endothelial nitric oxide synthase (eNOS) and neuronal NOS (nNOS) in the penis. Loss of the functional integrity of the endothelium and subsequent endothelial dysfunction plays an integral role in the occurrence of ED. This chapter reviews the use of two gene transfer techniques: adenoviral gene transfer of eNOS and eNOS gene-modified rat marrow stromal cells (rMSCs) to the penis of aged rats for the potential application of gene therapy for the treatment of ED.