Over the past three decades, a large body of work has been directed at the development of therapeutic cytokines. Despite their central role in immune modulation, only a handful of cytokine therapeutics has achieved regulatory approval. One of the major challenges associated with the therapeutic use of cytokines relates to their short serum half-life and low bioavailability. High doses are required to overcome these problems, which often result in dose-limiting toxicities. Consequently, most cytokines require protein engineering approaches to reduce toxicity and increase half-life. For this purpose, PEGylation, fusion proteins, antibody complexes and mutagenesis have been utilized. Here, we summarize past, recent and emerging strategies in this area.