The most prominent exemple of European economic integration is the European Union Single market, which aims to enable all participants to move freely by removing barriers and implementing existing rules. The free movement of persons, services, goods and capital offers individuals life, work, education in any Member State, easier business operations beyond the borders of their home country for business, while for consumers this means creating competition between producers resulting in lower prices for goods and services. The full potential of the Single market has not yet been realised due to the numerous barriers and harmful effects of market-strong undertakings. The paper sets out the elements of distortion of competition by examining the case-law for the conduct concerned within the European Union. It primarly defines the concept of an undertaking which, by its action on the market, affects trade between Member States. There is no uniform definition of the concept of undertaking, but it is sought to explain in the first part of this work through several cases and judgments of courts. The EU Courts shall give priority to the functional definicion ot the term, taking into account the activities of a specific undertaking. Examples of case-law cited in the work are regarded as an undertaking by any entity providing services and goods on the market, whether or not they operate within the public authority. In order to classify such an action as harmful, the relevant market is defined as a field of business and only after it is a dominant position which distorts competition if it is abused. This is also the main point of this paper, explaining that the very dominant position of an undertaking is not harmful, but is harmful abuse of such a position. The abuse is explained in more detail through direct or indirect imposition of discriminatory prices, as well as other unfair trading conditions, undermining markets trends or technical development and innovation to the destriment of consumers, weakening competition by applying unequal conditions in each other's operations, making the other party subject to unjustified and aggravating contractual clauses. After the analysis of the mechanisms of abuse of dominant position han been completed, the cooperation between national ana EU bodies in the implementation of antitrust policy and their efforts to preserve market cohesion are mentioned.