Liver-directed gene therapy represents a promising new modality for the treatment of inherited and acquired liver diseases. Clinical trials of liver-directed gene therapy are underway for diseases such as FH, OTC deficiency, and cancer. The main obstacles to effective gene therapy are the limitations of present gene delivery technology to express a desired gene safely and stably at therapeutic levels. With improved gene delivery technology and refinements in the ex vivo and in vivo approaches, a truly useful clinical tool will emerge.