We develop a model of a financial institution to study how shareholder—debt holder conflicts interact with prudential capital regulation and accounting measurement rules. Our analysis highlights the result that, for highly leveraged financial institutions—when prudential regulation play an important role—debt overhang and asset substitution inefficiencies work in opposing directions. We demonstrate that, relative to the “historical cost” regime in which assets and liabilities on an institution’s balance sheet are measured at their origination values, fair value could alleviate the inefficiencies arising from asset substitution, but exacerbate those arising from underinvestment due to debt overhang. The optimal choices of accounting regime and prudential solvency constraint balance the conflicts between shareholders and debt holders. Under fair value accounting, the optimal solvency constraint declines with the institution’s marginal cost of investment in project quality and the excess cost of equity capital relative to debt capital. Fair value accounting dominates historical cost accounting provided the solvency constraints in the respective regimes take their optimal values. If the solvency constraints are sub-optimally chosen, however, historical cost accounting could dominate fair value accounting.