1. This review describes experimental approaches to test the feasibility of using gene therapy to administer insulin to type 1 and type 2 diabetic patients. Two approaches, i.e. ex vivo and in vivo transfer of the insulin gene, are described. 2. Substantial progress has been made in recent years in engineering glucose-responsive beta-cell lines that have been genetically engineered to proliferate or differentiate in response to appropriate extracellular signals. 3. Non-beta-cell lines have been engineered to constitutively secrete insulin at a constant rate. These cells may improve glycaemic control in patients over longer periods when used in combination with insulin injections. Engineering glucose-stimulated insulin secretion in such cells has proved extremely difficult and several genes may he required. 4. In vivo transfer of the insulin gene to animals results in improved control of diabetes. However, for safety reasons this approach may have limited use in the treatment of diabetes in humans.