The European Union (EU) is a specific international organization with the power to adopt decisions that are binding not only to member states, but also to natural and legal persons on its territory. In addition, the EU is empowered to adopt binding acts in a vast number of areas that were exclusive domain of sovereign states in the near past. These characteristics make the legal system of the EU very specific, very developed and hence very complex. This is the reason why countries aspiring to be members of the EU must approximate their legal systems with the legal system of the EU, as a precondition that enables them to become member states. Approximation of legislation is the only way to enable the candidate country to take full advantage of all rights and honor all commitments that membership brings. The scope of this dissertation encompasses the process of approximation of the legal system of a country aspiring to become a member state of the EU, until the moment of accession. On the other hand, it does not cover the process of approximation after the accession to the EU, because then the process develops different characteristics, dynamics and methodology. General hypothesis of the dissertation that is being proven is that a country in the process of accession applies the law of the European Communities or acquis communautaire, before the moment of its official accession to the EU. In other words, the process of approximation is, in fact, the process of application of the acquis communautaire before joining the EU. Acceding country must prove its willingness and capability to implement acquis communautaire in order to become a member of the EU. That must be proven in the course of accession, before joining the EU. Speaking de iure, until accession, a country implements its own legislation and acquis communautaire as foreign law, but de facto, through approximation the country implements acquis communautaire disguised in the form of national legal acts. The dissertation aims to prove that proper implementation of the acquis communautaire in the legal order of the acceding country is a condition for membership and not a result of the accession. Author defines which part of acts adopted by the EU constitute the need for legal approximation that require legislative activity of the country, i.e.what is acquis communautaire which falls under the scope of this work. Author also defines, but doesn’t examine in the scope of this dissertation, which part of those acts create an obligation of the country to harmonize its politics and policies with EU politics and policies, creating a political obligation of the acceding country, but not a legal one. The author explores further in which political and economical context this approximation is being done and concludes that, even though approximation constitutes the most challenging condition for accession, it is only a part of the wider context of the overall harmonization of a country with the system of the EU in all areas under EU competences. This wider context is represented by the criteria for membership defined by the European Council conclusions adopted in Copenhagen in 1993 and in Madrid in 1995. Analyzing the process of approximation of legislation of a country in different stages of the process of accession, the author defines three deferent phases with different features. These phases are determined by the level of development of contractual relations of that country with the EU and by closeness of membership. He describes the following phases: the phase before a country signs a Stabilization and Association Agreement, when there is no obligation of a country to approximate its legislation with the acquis communautaire; the phase that starts with the signing of a Stabilization and Association Agreement, when a country takes on an obligation to approximate its legislation in the areas defined by the Agreement, and according to the timetable defined by the Agreement; and the phase after the accession negotiations are opened, when, in the course of negotiations, two sides define when and how the acceding country will fully approximate its legislation with the complete corpus of acquis communautaire. Further on, the author analyzes steps in the process of approximation in a specific legal area that determine the progress achieved in reaching full approximation. He concludes that there are three steps in the process. The first step is adoption of the legal act transposing the acquis communautaire into the national legal order, the second step is implementation of that act primarily by the executive bodies, securing its implementation in line with the practice and principles of the EU, and the third step is enforcement by the judiciary, thus giving the final proof that a country has approximated its legal system (legislative, executive and judicial branches) with the rules of the acquis communautaire. Examining the process of approximation of the legal system of Republic of Serbia with the acquis communautaire in the area of competition law, the dissertation shows that the term “approximation with the acquis communautaire” is, in fact, a legal standard, whose content is defined according to the specificities of an acceding country. Examining this particular area, author concludes that there are two sets of rules with which a country has to approximate its legal system. The first set of rules are those from which there can not be any deviation, while with the second set of rules a country could, or even has to, take into consideration its own specificities. In the first group, the author places rules on restrictive agreements and abuse of dominant position and in the second group rules on control of concentrations and administrative framework for the implementation of policy and legislation. By comparing legal systems of several countries, both member states and acceding countries, the author demonstrates that when approximating its legislation with the first set of rules, an acceding country does not have room for deviating from rules defined by articles 101 and 102 of the Treaty on Functioning of the EU. But, when approximating its legal system with the second set of rules, this possibility arises. Control of concentrations on a market is directly linked to its size and thresholds for control must be defined accordingly. The methodology of defining these thresholds is also variable and depends on the policy orientation of the legislator, with EU giving guidelines in the form of best practices. This draws to the conclusion that the main criteria determining the level of approximation in this area is empirical, namely it is the answer to the question: are concentrations effectively controlled in the market, or not. This, also, stands when speaking about institutional setups for implementing the competition policy and legislation in a country. Every country has to build up its own institutional setup according to its capabilities, needs, existing institutional framework and administrative traditions. This leaves enough maneuvering room for countries to reach the common objective of “full approximation with the acquis communautaire” using different methods and tools. The hypothetical-deductive method was used as a general scientific method of the dissertation. Comparative analysis method was also used, as a useful method for analyzing similarities and differences of experiences and actions of different countries that were or still are in the accession process. Analysis of the content of relevant documents and primarily their qualitative analysis was very important for conducting the research on which the dissertation is based upon. Evropska unija (EU) predstavlja specifičnu međunarodnu organizaciju sa pravom da donosi odluke koje su obavezujuće, ne samo za države članice nego i za pravna i fizička lica na njenoj teritoriji. Pri tom, Evropska unija je ovlašćena da donosi obavezujuće akte u velikom broju oblasti koje su u prošlosti bile regulisane isključivo nacionalnim pravnim aktima država. Ove karakteristike čine pravni sistem Evropske unije vrlo specifičnim i vrlo razvijenim, a samim tim i složenim. Zbog toga sve države koje pretenduju da postanu članice Evropske unije, moraju uskladiti svoje zakonodavstvo sa pravnim pravilima koja važe u poretku Evropske unije, a što je uslov za njihov prijem u članstvo ove organizacije. Usklađivanje zakonodavstva države koja pristupa jedini je način da ta država, nakon stupanja u članstvo EU, bude sposobna da preuzme u punoj meri prava i obaveze koje članstvo u Evropskoj uniji donosi. Predmet disertacije je proces usklađivanja zakonodavstva jedne države u procesu pristupanja Evropskoj uniji, do momenta stupanja u članstvo. Sa druge strane, predmet disertacije nije i proces usklađivanja nakon pristupanja EU, budući da tada proces usklađivanja dobija drugačije karakteristike, dinamiku i metodologiju. Opšta hipoteza koju disertacija potvrđuje jeste da država koja pristupa Evropskoj uniji primenjuje pravne tekovine evropskih zajednica, tj. acquis communautaire i pre momenta zvaničnog stupanja u članstvo. Drugim rečima, proces usklađivanja zapravo predstavlja proces primene acquis communautaire i pre stupanja u članstvo EU. Država koja pristupa EU mora pokazati da je sposobna da primenjuje acquis communautaire kako bi postala članica EU. To se mora dokazati u toku procesa pristupanja, dakle pre momenta samog stupanja u članstvo. Govoreći de iure, do momenta stupanja u članstvo Evropske unije, država primenjuje nacionalno zakonodavstvo, dok acquis communautaire za nju predstavlja strano pravo, dok de facto putem usklađivanja sa acquis communautaire država primenjuje to pravo, ali kroz formu nacionalnih propisa koji su prethodno usklađeni sa acquis communautaire. Cilj disertacije je da pokaže da je ispravna primena acquis communautaire u pravnom poretku države koja pristupa Evropskoj uniji zapravo uslov za članstvo, a ne posledica učlanj