This introductory chapter, written by two experts—one from an old Member State and the other of a new one, scheds light on process of European Union enlargement, particularly the accession of Central and Eastern European (CEE) states after the fall of the Iron Curtain in 1989. This historical event transformed both the political and social landscape of Europe. The chapter focuses in more detail on several attitudes regarding EU the englargment, which was initially viewed as a utopian vision promoting unity, the idea of a united Europe faced substantial practical challenges during the accession process. As former socialist states endeavored to align with EU standards, they were required to undertake significant political, economic, and cultural reforms which often led to social disruptions and economic imbalances. The old EU Member States perceived this expansion as mainly strategic, expecting the new entrants to comply with existing rules without considering the extensive transformation necessary for these nations to reach compatibility. While the accession offered CEE states a pathway to integration and the benefits of EU membership, it also entailed a lengthy and arduous process of adaptation to EU law and standards. The implications of enlargement extended beyond the CEE states, prompting a re-evaluation of labour and social rights within the EU as a whole. The chapte concludes, that accession allowed for an expansion of the EU's social profile, leading to stronger labour and social security legislation driven by human rights considerations. New Member States increased the complexity of EU decision-making, and their diverse experiences reshaped legislative priorities.