Tissue-resident macrophages comprise heterogeneous populations with unique functions and distinct gene expression signatures. While it has been established that they mostly originate from embryonic progenitors, the signals inducing a characteristic tissue-specific differentiation program remain unknown. Here we identify PPAR as the crucial transcription factor determining perinatal alveolar macrophage (AM) development and identity. Development of the fetal monocyte derived AM precursor was largely abrogated in CD11c-Cre/Ppargfl/fl mice. To reveal the underlying changes in gene expression, we performed microarray analysis of sorted WT and KO AM and pre-AM from 3 different timepoints.