Heme oxygenase-1 (HO-1) is regarded as a sensitive and reliable indicator of cellular oxidative stress. Studies on carbon monoxide (CO) and bilirubin, two of the three (iron is the third) end products of heme degradation have improved the understanding of the protective role of HO against oxidative injury. CO is a vasoactive molecule and bilirubin is an antioxidant, and an increase in their production through an increase in HO activity assists other antioxidant systems in attenuating the overall production of reactive oxygen species (ROS), thus facilitating cellular resistance to oxidative injury. Gene transfer is used to insert specific genes into cells that are either otherwise deficient in or that underexpress the gene. Successful HO gene transfer requires two essential elements to produce functional HO activity. Firstly, the HO gene must be delivered in a safe vector, e.g., adenoviral, retroviral or leptosome based vectors, currently being used in clinical trials. Secondly, with the exception of HO gene delivery to either ocular or cardiovascular tissue via catheter-based delivery systems, HO delivery must be site and organ specific. This has been achieved in rabbit ocular tissues, rat liver, kidney and vasculature, SHR kidney, and endothelial cells [Abraham et al., 1995a; Abraham et al., 1995b; Abraham et al., 2002c; Quan et al., 2004; Sabaawy et al., 2000; Sabaawy et al., 2001; Yang et al., 2004]. In this review, we discuss the functional significance of the HO system in various pathophysiological conditions and the beneficial therapeutic applications of human HO gene transfer and gene therapy in a variety of clinical circumstances.