Cytokines encompass a wide variety of proteins that can trigger many cellular activities. An important set of cytokines modulate inflammatory responses (inflammatory cytokines). These molecules have potent biological activities and have been a major focus for protein drug development. There have been both successes and failures in this area. Initial hurdles, such as limited manufacturing capacity, have now been largely overcome. However clinical development remains a challenge. On the basis of the history of cytokine therapeutics, a number of strategies for future drug development are considered.