The objective of the monograph was to show the position of the Sejm of the Republic of Poland as one of the national parliaments of the European Union member states. This significant and worthy of deep analysis issue is directly connected to the necessity of determining place of national parliaments in the institutional system of the European Union. Complex and multilevel structure of the European Union assumes cooperation of numerous institutions, located at various decision-making levels – both transnational and national, as well as regional and local. This cooperation must be realised in vertical and horizontal dimension. In this complex decision-making system, the role of national parliaments was strengthened by provisions of the Treaty of Lisbon. The monograph is an attempt at synthetic review of the role, which the Sejm of the Republic of Poland can (and should) play, as a chamber of a national parliament of a member state, in the integration processes.