The purpose of this thesis is to explain and analyze the way competition law is implemented in the Republic of Croatia by the Agency for the Protection of Market Competition when prohibited horizontal agreements are concluded, as well as the judicial control over its decisions. With the accession of the Republic of Croatia to the European Union, it was necessary to harmonize the national legislation with the legal acquis of the European Union in order to ensure unhindered competition on the internal market for entrepreneurs. In this paper, we will deal with the issue of the implementation of competition rules both by the Agency for the Protection of Market Competition and judicial supervision, and we will try to find an answer to the question of which factors affect the insufficiently developed implementation of competition law. Through the conducted analysis, we will come to the conclusion that despite the systematic application of competition law by the Agency for the Protection of Market Competition, the number of decisions is still relatively low, especially if we also take into account the fact that there is no clear standard of evidence for determining the existence of a cartel, which the result is the non-confirmation of already rare cartel decisions. When relatively low fines are added to that, we come to the conclusion that the enforcement of market competition law in relation to cartels is not a deterrent to other entrepreneurs. At the level of the judicial instance, the insufficient professionalism of judges can be seen as a problem, which can be seen in the sometimes wrong interpretation of national regulations, but also in the application of the acquis of the European Union and the practice of the Court of the European Union. These conclusions are derived from a cross-section of the implementation of competition law in the Republic of Croatia in the last 10 years or so, but also from a detailed analysis of several important cartel cases.