The main theory of capital structure—the Brusov–Filatova–Orekhova (BFO) theory replaced the famous theory of cost and capital structure of Nobel laureates Modigliani and Miller. The authors departed from the Modigliani-Miller assumption of perpetuity (infinite life-time) of companies and developed a quantitative theory of assessing the main parameters of financial activity of companies with an arbitrary age (and an arbitrary life-time). The results of the modern BFO theory turn out to be completely different from those of the Modigliani-Miller theory. It shows that latter, due to its perpetuity, underestimates the estimate of the weighted average cost of capital and the cost of equity capital of the company and significantly overestimates the estimate of the company’s capitalization. Such an incorrect assessment of the key performance indicators of the financial activities of companies led to underestimation of risks, impossibility or serious difficulties in making adequate management decisions, which became one of the implicit causes of the global financial crisis of 2008. A lot of qualitatively new effects have been discovered in the BFO theory and which are absent in the Modigliani-Miller theory. In the BFO theory, when investing at certain values of investment return, there is an optimal investment structure. The authors also developed a new mechanism for forming the optimal capital structure of a company, which differs from that proposed by the trade-off theory, the bankruptcy of which was proven by the authors of the BFO theory.