Granulocyte-macrophage colony stimulating factor (GM-CSF) is a growth factor first identified as an inducer of differentiation and proliferation of granulocytes and macrophages derived from haematopoietic progenitor cells. Later studies have shown that GM-CSF is involved in a wide range of biological processes in both innate and adaptive immunity, with its production being tightly linked to the response to danger signals. Given that the functions of GM-CSF span multiple tissues and biological processes, this cytokine has shown potential as a new and important therapeutic target in several autoimmune and inflammatory disorders — particularly in rheumatoid arthritis. Indeed, GM-CSF was one of the first cytokines detected in human synovial fluid from inflamed joints. Therapies that target GM-CSF or its receptor have been tested in preclinical studies with promising results, further supporting the potential of targeting the GM-CSF pathway. In this Review, we discuss our expanding view of the biology of GM-CSF, outline what has been learnt about GM-CSF from studies of animal models and human diseases, and summarize the results of early phase clinical trials evaluating GM-CSF antagonism in inflammatory disorders.