The document discusses the evolution of thinking around corporate capital structures and optimal debt levels. It notes that early economic models assumed rational behavior and perfect markets in determining optimal capital structure. However, Miller argued that a firm's value will be independent of its capital structure even when accounting for interest tax deductibility. The paper examines how bankruptcy costs, taxes, and market equilibrium impact capital structure decisions. It also notes some of the assumptions in Miller's models, such as personal tax rates on stock and bond income.