Due diligence is defined as a set of tasks, that leads into a thorough review of the company in order to take a decision on the intended transaction. Although it is based on past business, the results give the subscriber a clear picture of the company's future. Considering that the due diligence results usually tip the balance and persuades the buyer into the purchase, or discourages it from it, it represents one of the most important steps (if not the most important) in the process of buying a company. Regardless of its importance, in most jurisdictions the institute is not governed by a positive law. Consequently, many people do not fully understand its significance, or they do not know how to perform it. In this master's thesis, the author wishes to solve these questions. In doing so, it focuses primarily on legal due diligence in the company's acquisition procedures.At the beginning of her master's thesis, the author defines the concept of due diligence and represents its main characteristics. After that she focuses on the legal due diligence in the procedures for purchasing the company. In this context she wants to practically present the course and the content of legal due diligence, which leads the buyer to the right decision on the intended investment. Throughout the master's thesis, the author highlights the multitude of issues arising from the institute. These are issues related to its authorization (who has jurisdiction to adopt the decision about requested due diligence, when can the authority reject the request for due diligence, does the buyer has a right to preform due diligence), the problem of disclosing information that are defined as trade secrets and those that are defined as internal information by the Financial Instruments Market Act, and the questions of the responsibility of the persons involved in due diligence. The author also devotes attention to comparison between a due diligent review in a joint stock company and a due diligent review in a limited liability company and explains the reasons for the existing differences. Her objective is to present the general arrangement of the course and the content of the institute, to clarify the dilemmas that have arisen in connection with the due diligence, and to check if the institute could to some extent, also be included in a positive law.