Doubtless is the fact that Poland, in comparison with other not only western countries, still remains delayed in terms of the development. This results mainly from the Polish structural problems whose origins can be found in the country’s history. The restructurization processes of industry and agriculture together with a slow privatization of these branches of economy, despite visible positive results, brought about threats of unemployment, a low investment ability, little competitiveness of enterprises or ineffectiveness of the institutional system. What is more, the above-mentioned dangers and those which have not been cited occur to different degrees in different regions. Therefore, a low socio-economic cohesion is itself an obstacle to the enlargement of the cohesion at the European level. Despite the progressing outer convergence, it can be stated that, with the exception of a few big cities and few areas attractive in terms of tourism and agriculture, all Polish voivodships are threatened by the peripheralization in the integrating Europe. Such processes are especially visible in the eastern areas of Poland. A typical way of thinking where the EU is supposed to eliminate the risk of marginalization of the Polish regions implies another danger. It consists in being dependent on the outer help, no matter what the ability to implement the received financial support is. Thus, the actions on different levels must be taken not only to decrease the above-mentioned dangers of marginalization but also to facilitate better conditions for a further development of the regions. Therefore, it must be remembered that every enlargement of the European structures has had a bigger or smaller influence on the inner cohesion of the EU. The countries which are socio-economically less developed have been accepted as members of the integration process. On 1 May 2004, ten new countries became members of the EU, including Poland. As this was the biggest enlargement in the history of the EU, it has influenced both the institution and the economy of this organization, but first and foremost, the inner coherence of the EU.