The paper aims to analyse selected legal and economic issues connected with control of concentration in the banking sector in Poland. The antitrust law aims to counteract the monopolisation of the market. Undertakings, including banks, can take actions which lead to mergers or takeovers which can strengthen the market power of some of them. The antitrust authority has normative instruments to influence the market in order to counteract its monopolisation. These instruments usually concern control of the concentration of companies (also banks). In Poland, the antitrust authorities (the President of the Office of Competition and Consumer Protection ? UOKiK) also have a legal instrument which can counteract the anticompetitive actions of banks.Because of the specific features of the financial market and financial services, general characteristics of these issues are presented in the paper. Moreover, the methods of measurement and aims of control of concentration are analysed. The main legal instrument of limiting the concentration is connected with an administrative procedure. Therefore, general provisions of these procedures are also shown in the paper. To illustrate the above-mentioned theoretical issues the statistical data concerning the level of concentration in the Polish banking sector is also analysed.