Despite the critical importance of alveolar function for overall health and the significant morbidity and mortality attributable to diseases causing acute and chronic respiratory failure in infants and adults, little is known, or at least agreed upon, about how alveoli maintain and repair themselves throughout life. We recently used clonal analysis to map the entire life cycle of individual AT2 cells in vivo, and identified a rare subset that exhibited stem cell properties, including multi-potency, self-renewal, and activity throughout the lifespan. Here, we propose to build upon this foundational work to answer important outstanding questions about the cellular and molecular mechanisms of alveolar maintenance and stem cell regulation. Our approach employs some of the most novel and cutting-edge genetic and single cell genomic technology to accomplish the ambitious aims. Specific Aims are: A) to determine the extent and cellular mechanism of physiologic alveolar epithelial renewal in vivo by Aa) quantifying nuclear label retention by AT2 cells and Ab) measuring AT2 cell clonal fate dynamics during ageing, B) to empirically characterize molecular heterogeneity within the AT2 cell population and identify markers that distinguish AT2 stem cells Ba) using single cell RNA-sequencing of AT2 cells and Bb) by bulk cell RNA-sequencing of label-diluting versus label-retaining AT2 cells (identified in Aim Aa), C) to directly test if signaling via EGFR regulats AT2 stem cell function in vivo and whether repetitive ablation of EGFR-deleted lung epithelial cells results in alveolar enlargement or pulmonary fibrosis. By employing cutting edge technology and approaches to understand how the gas exchange surface of the lungs remain healthy throughout life, and how new cells are made to replace those that are lost from injury, we are paving the way towards developing medical or cell-based treatments to treat devastating and fatal lung diseases like emphysema and infant respiratory distress syndrome.