In humans, treatment for inborn errors of liver metabolism has focused on dietary, drug, and cell therapies. However, significant morbidity and mortality still remain and alternative and/or adjunctive strategies are needed. It has been 15 years since human gene therapy trials were initiated for genetic diseases. While significant progress has been made in the preclinical arena in a variety of disease models, sustained effect in humans has still eluded the field of inborn errors of liver metabolism. At this time, achievement of clinical efficacy in different animal models has been reported with multiple gene transfer technologies. Current efforts are aimed at fully understanding host-vector interactions, and how manipulation of these interactions can help us to increase the therapeutic index of any specific therapy. As with any therapy, it will be the balance of this index and the disease natural history which will determine the future of clinical studies and their outcomes.