Accession of new Member States to the European Union is provided for in Article 49 of the EU Treaty (TEU). The Council must agree unanimously to open negotiations, after consulting the Commission and receiving the assent of the European Parliament. The conditions of admission, any transition periods and adjustments to the Treaties on which the Union is founded must be the subject of an agreement between the applicant country and the Member State. To enter into force, the agreement requires ratification by all the contracting states in accordance with their respective constitutional requirements.
To accommodate the Central and Eastern European countries’ interest in joining the EU, the Copenhagen European Council developed the so-called Membership (also known as the Copenhagen) Criteria. This requires potential Member States to fulfil certain parameters relating to democratic stability, human rights, the rule of law and the existence of a market economy, etc. Gaining EU membership entails a detailed process of 1) screening a country's eligibility (negotiations between the Commission and candidate country); 2) a monitoring and review procedure of progress being made; and finally 3) a ratification process. To assist candidate countries in becoming full EU members, the Directorate-General for Enlargement offered various financial packages, which have subsequently been consolidated into the Instrument for Pre-Accession in 2006. On 1 May 2004, the accession of 10 new Member States (the Czech Republic, Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia) took place.
Membership in 2007 of Bulgaria and Romania was given the green light following the Commission’s announcement in September 2006 that both countries had made tremendous strides in meeting the accession criteria. Olli Rehn, Commissioner for Enlargement, noted that Bulgaria and Romania had demonstrated an ability to reform their judicial processes, fight corruption, especially amongst officials, as well as support moves to lift the immunity of members of parliament. In October 2005, the Commission entered into accession negotiations with Croatia after the International Criminal Tribunal for the former Yugoslavia indicated that the country was now fully cooperating in assisting it in its work. .
At the European Council on 17 and 18 June 2004, the Union reaffirmed its commitment to start accession negotiations with Turkey without delay, provided it complied with the Copenhagen political criteria. In its recommendation of 6 October 2004, the Commission concluded that Turkey satisfied the Copenhagen political criteria and recommended that accession negotiations be opened. On 3 October 2005, the EU started negotiations with Turkey.
In October 2010, Croatia, Iceland and the Former Yugoslav Republic of Macedonia have also gained the status of candidate countries..
The industrial relations systems of the Member States which joined in May 2004 and of the future acceding countries will be confronted with the emerging patterns of industrial relations in the EU15 countries (the 15 Member States pre-May 2004). This process will affect such general characteristics in these countries as: heterogeneous and fragmented trade unions and employer organisations; limited scope of collective bargaining, with systems becoming more decentralised and operating mainly at company level; underdeveloped sectoral level bargaining; low levels of collective bargaining coverage, and meagre content of collective agreements; the widespread absence of works councils; the lack of social dialogue in the public sector; and an asymmetrical tripartitism, with strong governments confronting weaker social partners.
Enlargement therefore brings with it consequences for employment and industrial relations in both the acceding countries and in the Member States, exercising an unpredictable influence on the evolution of social policy at EU level.