The document discusses capital structure, defined as the proportion of long-term financing sources in a firm's total capital, and the criteria a financial manager uses to select these sources. It reviews various theories of capital structure, including Net Income, Net Operating Income, Traditional, and Modigliani-Miller theories, each addressing the relationship between debt and equity in influencing a firm's value and cost of capital. Key assumptions across these theories emphasize the role of stability in business risk and market perceptions, as well as the impact of debt levels on overall capital costs.