The understanding of cytokine networks and the exploitation of these networks for the treatment of immune and inflammatory diseases as well as cancer depend on in vivo delivery of cytokines. Due to instability of recombinant cytokine proteins, investigators have employed cytokine-encoding gene therapy vectors to induce high levels of cytokine expression in vivo. Numerous gene therapy vectors have been developed recently which are suitable for this purpose. Recent advances in the design of adenovirus, adeno-associated virus, poxvirus, retrovirus, lentivirus, and nonviral vectors are described here. Properties of the various vector systems which determine their usefulness for cytokine gene delivery are compared. The implementations of cytokine-encoding gene therapy vectors for analyzing immune responses and for the therapy of inflammatory disorders, immune disease, infections and cancer are reviewed.