Since bank credit lines are a major source of corporate funding and liquidity, we examine the determinants of credit line usage with a database of Spanish corporate credit lines. A line's default status is the primary factor driving its usage, which increases as a firm approaches default. Several lender characteristics suggest an important role for bank monitoring in firms' usage decisions. Credit line usage is found to be inversely related to macroeconomic conditions. Overall, while several factors influence corporate credit line usage, our analysis suggests that default and supply-side variables are the most important.