Polish businesses have faced a number of challenges in connection with the country’s membership of the European Union. First, companies must make efforts to increase their international competitiveness; second, they must make their operations more international in nature. The question is if Polish businesses can be supported in their efforts to improve their international competitiveness, while not deforming the market mechanism. Any form of supporting businesses should have a positive influence on their internationalization behaviors. A cluster theory put forward by American economist Michael Porter could provide inspiration in the search for ways to increase the international competitiveness and internationalization of Polish enterprises, the authors say. Porter argues that clusters have the potential to affect competition in three ways: by increasing the productivity of the companies in the cluster; by driving innovation in the field; and by stimulating new businesses in the field. Porter’s cluster theory is at the center of a survey conducted by Gorynia and Jankowska among enterprises in Poland’s Wielkopolska region in July and August 2006 and in April and May 2007. These surveys represent the principal part of a project dedicated to the role of clusters in supporting the international competitiveness and internationalization of Polish enterprises. The authors describe the results of their empirical studies of three clusters located in Wielkopolska: a furniture cluster, a heating-boiler cluster, and an automotive cluster. The authors present the opinions of the companies surveyed regarding the influence of the emerging clusters on the competitiveness and internationalization of participating companies. The surveys were conducted in the form of structured direct interviews with company executives. The main research tool was a questionnaire with 21 questions. The results obtained by the authors show that the development of each cluster depends on the behavior of enterprises and the activities of business self-regulation organizations, in addition to economic policy. A key factor is the promotion of the positive effects of exchanging knowledge among cluster participants. Participation in a cluster strengthens the ties between companies and increases their competitiveness and internationalization, the authors conclude.