The Modigliani–Miller theory, discussed in Chap. 5 , has a lot of limitations. One of the most important and serious assumptions of the Modigliani–Miller theory is that all financial flows as well as all companies are perpetuity. This limitation was lift out by Brusov–Filatova–Orekhova in 2008 (Filatova et al., Bull FU, 48, 68–77, 2008), who have created BFO theory—modern theory of capital cost and capital structure for companies of arbitrary age. Despite the fact that the Modigliani–Miller theory is currently a particular case of the general theory of capital cost and capital structure—Brusov–Filatova–Orekhova (BFO) theory—it is still widely used at the West. In current chapter we discuss one more limitation of Modigliani–Miller theory: a method of tax on profit payments. Modigliani–Miller theory accounts these payments as annuity–immediate while in practice these payments are making in advance and thus should be accounting as annuity–due. We generalize the Modigliani–Miller theory for the case of advance tax on profit payments, which is widely used in practice, and show that this leads to some important consequences, which change seriously all the main statements by Modigliani and Miller. The modified Modigliani–Miller theory (MMM theory) is the second of the three main theories of capital structure and capital cost. In this chapter we describe shortly the main results of MMM theory.