The chapter examines the multifaceted impact of CEE countries’ accessions to the EU in 2004, 2007 and 2013 on labour market dynamics, with a specific focus on posted workers. Initially devised to manage sudden labour migrations to Western Europe, transitional arrangements encountered new challenges with the emergence of posted workers exempted from such restrictions. These workers swiftly became pivotal sources of affordable labour for Western employers, prompting inquiries into the competitive advantages enjoyed by both posted workers and companies from CEE countries. The analysis focuses on evaluating the positioning of CEE countries’ posted workers in the EU labour market and assessing the impact of EU legislation on national systems. Specifically, it scrutinises the implications of the 2018 revision of the Posted Workers Directive, introduced in anticipation of the European Pillar of Social Rights, which aimed to enhance protections for posted workers. However, not all member states supported this revision, as evidenced by the analysis of responses from CEE countries in the ‘yellow card’ procedure before the Commission. Besides that procedure, the paper evaluates the actions of Poland and Hungary before the CJEU amidst the contentious political debate surrounding this reform. The authors raise critical questions: How is EU legislation impacting the CEE national systems of posting and vice versa? Do the policy approaches adopted by CEE countries genuinely foster economic growth, or do they perpetuate existing structures? Should posted workers bear the brunt of competition for advantages in the EU labour market, or should mechanisms be implemented to prevent social dumping? Therefore, this paper aims to deepen understanding of the complex interrelation between EU enlargement, labour mobility, and the legal frameworks shaping the workforce in CEE countries. It underscores the necessity of balancing economic competitiveness with social cohesion to ensure equitable outcomes for all stakeholders involved.