The book introduces and discusses the modern theory of the cost of capital and capital structure - the BFO theory (Brusov-Filatova-Orekhova theory), which is valid for companies of arbitrary age and which replaced the theory of Nobel laureates Modigliani and Miller. The theory takes into account the conditions faced by companies operating in the real economy, such as revenue fluctuations; the arbitrary frequency of tax on profit payments (monthly, quarterly, semi-annual or annual payments), both for advance income tax payments and for payments at the end of the respective period; and the arbitrary frequency of interest on loans payments. The impact of these conditions on the company value, on the cost of raising capital, on the company's dividend policy and managerial decisions are discussed. The book subsequently develops new applications of the BFO theory in several areas such as corporate finance, corporate governance, investments, taxation, business valuations and ratings.