We study optimal capital structure by first estimating firm-specific cost and benefit functions for debt. The benefit functions are downward sloping reflecting that the incremental value of debt declines as more debt is used. The cost functions are upward sloping, reflecting the rising costs that occur as a firm increases its use of debt. The cost functions vary by firm to reflect the firm’s characteristics such as asset collateral and redeployability, asset size, the book-to-market ratio, profitability, and whether the firm pays dividends. We use these cost and benefit functions to produce a firm-specific recommendation of the optimal amount of debt that a given company should use. In textbook economics, equilibrium occurs where supply equals demand. Analogously, optimal capital structure occurs where the marginal benefit equals the marginal cost of debt. We illustrate optimal debt choices for specific firms such as Barnes & Noble, Coca-Cola, Six Flags, and Performance Food Group, among others. We also calculate the cost of being underlevered for companies that use too little debt, the cost of being overlevered for companies that use too much debt, and the net benefit of debt usage for those that are correctly levered. Finally, we provide formulas that can be easily used to approximate the cost of debt function, and in turn to determine the optimal amount of debt, for any given firm. van Binsbergen is from the Kellogg Graduate School of Management, Northwestern University, and the Graduate School of Business, Stanford University, and NBER; Graham is from the Fuqua School of Business, Duke University, and NBER; and Yang is from the McDonough School of Business, Georgetown University. This paper updates our paper “The Cost of Debt” published in the Journal of Finance (December 2010), with a particular focus on a practitioner audience.