One of the two main theories of the capital structure and capital cost is the theory of Nobel Prize winners Modigliani and Miller (The American Economic Review, 48, 261–297, 1958). In this chapter we describe shortly the main results of this theory, understanding which is absolutely necessary for following modification of existing rating methodologies. Under the capital structure, one understands the relationship between equity and debt capital of the company. Does capital structure affect the company’s main settings, such as the cost of capital, profit, value of the company, and the others, and, if affects, how? Choice of an optimal capital structure, i.e., a capital structure, which minimizes the weighted average cost of capital, WACC, and maximizes the value of the company, V, is one of the most important tasks solved by financial manager and by the management of a company. The first serious study (and first quantitative study) of influence of capital structure of the company on its indicators of activities was the work by Modigliani and Miller (The American Economic Review, 48, 261–297, 1958). Until this study, the approach existed (let us call it traditional), which was based on empirical data analysis.