It is 10 years since the first gene therapeutic was administered to patients. Since that time approximately 3500 patients have been treated and although some evidence of gene transfer has been seen it has generally been inadequate for a meaningful clinical response. The major challenges have been delivery of DNA to the target cells and duration of expression. A variety of gene transfer systems have been evaluated and these are summarized with their advantages and disadvantages. Encouraging data have been obtained recently by a number of workers and there is some optimism that the enormous potential of gene therapy will be realized in the second decade of its development.