The article discusses the theoretical concept of the cost of capital as well as the practical aspects of using information about the cost of capital in managing companies. The author also reviews the results of a survey among Polish companies on the cost of capital in decision-making processes. The cost of capital plays an important role in calculating the economic value of the company and its components as well as in measuring the net present value of investment projects. The cost of capital makes it possible to convert future expected cash flows from assets into the present value of these assets. Future cash flows are discounted using the cost of capital as a discount rate. The importance of the cost of capital in managing companies largely depends on to what extent companies use economic value based on discounted cash flows. Comparing the actual rate of return on invested capital with the weighted average cost of capital makes it possible to find out whether the company creates economic value added (EVA). The results of the survey carried out among Polish companies indicate that the concept of the cost of capital is understood in different ways. Only about half the surveyed companies use data on the cost of capital in feasibility studies, valuation, in the choice of sources of financing and in the evaluation of their own performance (for example, in EVA calculation).