Although cytokines are critical in maintaining normal physiology, excessive production of these proteins can lead to pathological consequences. Inhibitors of cytokine production or action are therefore widely investigated as potential therapeutic agents in a variety of immune and inflammatory diseases. Indeed, the successful application of inhibitors of tumor necrosis factor-alpha in rheumatoid arthritis and Crohn's disease heralds the great therapeutic potential of biopharmaceutical agents to counteract cytokine activities. Emerging opportunities for new therapeutics, as well as the challenges we face in their use and development, are described in this review.