The article examines how U.S. firms' borrowing changes over time and affects their growth and response to economic shocks. Private firms tend to borrow more as they grow, while public firms borrow less. Young private firms rely more on debt, while older ones switch to equity. During the Great Recession, private firms reduced their borrowing, but public firms did not. Private firms' growth is linked to borrowing, while public firms' growth is not. This suggests that private firms face financial constraints, especially during crises.