Capital Structure Decisions from Optimal Leverage to Financial Flexibility

Capital structure research began with a deceptively simple question: does the division of financing between debt and equity change firm value? The literature gradually widened that question by adding taxes, bankruptcy costs, agency conflicts, private information, market timing, adjustment costs, national institutions, managerial traits, debt design, and intangible capital. This narrative review examines how those additions changed both theory and practice. It draws on influential foundational papers and selected empirical studies from developed and emerging economies. The evidence does not identify one theory that explains every firm or period. Instead, leverage reflects several forces whose importance varies with cash-flow stability, assets, investment opportunities, access to lenders, institutional conditions, and managerial judgment. Recent work places particular emphasis on maturity, covenants, refinancing risk, unused borrowing capacity, and the ability to finance future investment. The review concludes that capital structure has shifted from a largely static search for one optimal debt ratio toward an adaptive financing policy. Classical theories remain useful, but their insights are now applied within a broader concern for resilience and financial flexibility.