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Background: MiR-154 is abundant in neonatal lungs. Experimental evidence for a direct effect of miR-154 on lung development is still lacking. Aims and methods: We aim to investigate the effect of miR-154 by using an in-vivo gain-of-function ( CCSPrtTA/rtTA;tet(O)miR-154/+ ) and an in-vitro loss-of-function (morpholino) approach pre- and postnatally. Results: First, the expression of miR-154 from E10.5-P2 was analyzed by RT-qPCR and FISH. MiR-154 expression increased dynamically from E10.5 and peaks at P2. Attenuation of miR-154 in WT embryonic lung (CD1) harvested at E11.5 and treated with morpholino vs. control reveals a significant decrease in lung branching after 48h of culture. Prenatal overexpression of miR-154 from E7.5-E18.5 in CCSPrtTA/rtTA;tet(O)miR-154/+ mice (lung epithelial specific overexpression) and analysis of the lung structure by using alveolar morphometry shows increased mean linear intercept (MLI) and alveolar airspace but decrease in septal thickness indicating alveolarization defects. RT-qPCR reveals a decrease in Fgf10 signaling as well as markers for alveolar myofibroblast but increase in the epithelial marker EpCAM. In contrast, postnatal overexpression of miR-154 by using CCSPrtTA/rtTA;tet(O)miR-154/+ from P0-P16.5 leads to increased Fgf and Tgf-β signaling in the mutant group as well as increased MLI without changes in airspace and septal thickness. Interestingly, pull-down assay reveals caveolin 1 (Cav1), which is responsible for Tgf-β/Tgf-βR degradation, as a target of miR-154 . Conclusions: MiR-154 plays a crucial role in branching morphogenesis and alveologenesis by mediating Tgf-β signaling through caveolin 1.