The European Union competition law covers, inter alia, company concentration control. It exerts a significant impact on European market structure changes, mostly due to the role played by the European Commission, whose task is to assess whether the planned merger would restrict effective competition on the Community market. In its assessments of consequences of concentration the European Commission puts special emphasis on evaluation of a proper market, due to the implied impact of mergers on effective competition. Under the competition protecting law, the proper market is a product market and a geographical market, in which mergers may involve consequences detrimental for competition. The European Commission also plays a relevant role in assessment of enterprise domination, which allows to state whether concentration would strengthen the dominating position on a proper market. The Commission also examines the possibility of occurrence of a joint dominating position allowing entrepreneurs to raise prices and to cut production, and this way to stifle competition. The European Commission evaluates economic consequences, which may arise from concentration and points out the advantages which may be obtained by consumers as a result of concentration.