Cytokines are small proteins with the properties of locally acting hormones. They are essential for the healing of connective tissues following injury. Various cytokines are expressed endogenously during the healing responses of those ligaments and tendons which possess them, and there is evidence that the exogenous application of certain cytokines promotes healing. Because healing is a multistep process, several different cytokines may need to be applied at different times to achieve a satisfactory response. Sustained, localised delivery of the appropriate cytokines presents a technical challenge to their clinical use in the repair of ligaments and tendons. Slow release devices and gene transfer are being evaluated as strategies for obviating this limitation.