Today, the main purpose of companies is to maximize the value of the company, which takes into account both the risk and time factor. This purpose necessitates taking decisions in a way that will maximize the market value of the company's stocks. One of the most important factors affecting the value maximization is the capital structure, in other words, the financing mix. Capital structure expresses the composition of resources used by companies to finance their assets. Considering the cost of each resource used in the financing of assets, the creation of an optimum capital structure for companies has a significant impact on firm value. Many theories have been developed in order to determine the optimum capital structure of companies. In this research, first of all, capital structure, capital cost and factors affecting capital structure are discussed in the theoretical framework, and then classical and modern capital structure theories are explained.