Tissue-resident macrophages shape the development and homeostasis of the organs that they colonize. Yu et al. (2017) demonstrate that, unique to the lung, the instructive cytokine TGF-β collaborates with GM-CSF to induce the master transcription factor PPARγ to drive alveolar macrophage differentiation and maintenance early in life. Tissue-resident macrophages shape the development and homeostasis of the organs that they colonize. Yu et al. (2017) demonstrate that, unique to the lung, the instructive cytokine TGF-β collaborates with GM-CSF to induce the master transcription factor PPARγ to drive alveolar macrophage differentiation and maintenance early in life.