We develop a dynamic tradeoff model to examine the importance of manager-shareholder conflicts in capital structure choice. In the model, firms face taxation, refinancing costs, and liquidation costs. Managers own a fraction of the firms' equity, capture part of the free cash flow to equity as private benefits, and have control over financing decisions. Using data on leverage choices and the model's predictions for different statistical moments of leverage, we find that agency costs of 1.5% of equity value on average are sufficient to resolve the low-leverage puzzle and to explain the dynamics of leverage ratios. Our estimates also reveal that agency costs vary significantly across firms and correlate with commonly used proxies for corporate governance.