The study of the quantitative impact of taxes on the main financial indicators of companies (microeconomic level) and the development of recommendations for the regulator on taxation issues (macroeconomic level) requires the use of modern theories of capital structure: Brusov–Filatova–Orekhova (BFO) and Modigliani–Miller (MM) ones. This Chapter examines the state of the capital structure and capital cost theory from the middle of the last century, when the first quantitative theory was created, to the present. The two main theories Modigliani–Miller (MM) and Brusov–Filatova–Orekhova (BFO) are discussed and analyzed, as well as their numerous modifications and generalizations. Also discussed is the latest stage in the development of the theory of capital structure, which began a couple of years ago and is associated with the adaptation of two main theories of capital structure (Brusov–Filatova–Orekhova and Modigliani–Miller) to established practice of the functioning of companies. This generalization takes into account the real conditions of the work of the companies. It was noted, that taking into account some effects that are present in economic practice (such as variable income, frequent payments of tax on income, advance payments of tax on income etc.) brings both theories closer, and even the Modigliani–Miller theory, with all its many limitations, becomes more applicable in economic practice. However, it should be remembered that the Modigliani–Miller theory is only true for perpetual companies, while the BFO theory is valid for companies of any age and from this point of view they never coincide.