The role of granulocyte-macrophage colony-stimulating factor (GM-CSF) was sequentially redefined during the past decades. Originally described as a hematopoietic growth factor for myelopoiesis, GM-CSF was recognized as a central mediator of inflammation bridging the innate and adaptive arms of the immune system. Phagocytes sensing GM-CSF adapt an inflammatory phenotype and facilitate pathogen clearance. However, in the context of chronic tissue inflammation, GM-CSF secreted by tissue-invading lymphocytes has detrimental effects by licensing tissue damage and hyperinflammation. Accordingly, therapeutic intervention at the T cell-phagocyte interface represents an attractive target to ameliorate disease progression and immunopathology. Although GM-CSF is largely dispensable for steady state myelopoiesis, dysregulation, as seen in chronic inflammatory diseases, may however lead to disrupted haematopoiesis and long-term effects on bone marrow output. Here, we will survey the role of GM-CSF during inflammation, discuss the extent to which GM-CSF-secreting T cells, debate their introduction as a separate T cell lineage and explore current and future clinical implications of GM-CSF in human disease settings.