The purpose of this article is to clarify doubts arising in the context of the agreement on maintaining the register of shareholders of a simple joint-stock company and a joint-stock company. The conclusion resulting from the considerations contained in the article comes down to, among others, the following conclusions: 1) The register of shareholders has a special character – resulting from the principle of constitutive entry in the register and the principle of security and certainty of trading – which determines the limits of the contractual freedom of the parties to the agreement on maintaining the register of shareholders and, consequently, the content of the legal relationship created on its basis. 2) A resolution of the shareholders containing consent to conclude an agreement with a specific entity responsible for maintaining the register is a legal condition (conditio iuris), a condition necessary for the effective conclusion of the agreement. This consent cannot be of a blank nature. 3) It is unacceptable to conclude a conditional agreement on maintaining the register, the effectiveness of which would depend on the adoption of a resolution of the shareholders. 4) The entity maintaining the register of shareholders of a company cannot be a shareholder of that company. 5) The parties to the agreement on maintaining the register of shareholders may not expand the register data unless the company’s articles of association (statute) provide for it.