The most recent enlargement of the European Union saw Croatia become the European Union's 28th member state on 1 July 2013. The country applied for EU membership in 2003, and the European Commission recommended making it an official candidate in early 2004. Candidate country status was granted to Croatia by the European Council in mid-2004. The entry negotiations, while originally set for March 2005, began in October that year together with the screening process.

The accession process of Croatia was complicated by the insistence of Slovenia, an EU member state (since 2004), that the two countries' border issues be dealt with prior to Croatia's accession to the EU. Croatian public opinion was generally supportive of the EU accession process, despite occasional spikes in euroscepticism.

Croatia finished accession negotiations on 30 June 2011, and on 9 December 2011, signed the Treaty of Accession. A referendum on EU accession was held in Croatia on 22 January 2012, with 66% of participants voting in favour of joining the Union. The ratification process was concluded on 21 June 2013, and entry into force and accession of Croatia to the EU took place on 1 July 2013.

Accession requirements included: judicial reform to strengthen the independence, accountability, impartiality, professionalism and efficiency of the judiciary; a crackdown on corruption and organised crime (this resulted in the conviction of former Prime Minister Ivo Sanader for taking bribes); strengthen the protection of minorities; settle outstanding refugee return issues; improve the protection of human rights.

Croatia had to extradite several of its citizens to the International Criminal Tribunal for the Former Yugoslavia (ICTY), a United Nations body, an issue that was often contentious in domestic politics. Croatia's relations with the ICTY had continually been cited by the EU officials as something that required further improvement. Ratification of the EU Stabilisation and Association Agreement with Croatia had been stalled because of this.

The European Council, after its summit of 20 December 2004, set the following 17 March as the date to start entry negotiations, provided that Croatia continued to co-operate fully with the ICTY. On 16 March 2005, the day before talks were to begin, the EU postponed the commencement of negotiations, because the ICTY prosecution assessed the Croatian efforts to capture the fugitive general Ante Gotovina (indicted by the ICTY for war crimes and crimes against humanity, but at large since 2001) as neither timely nor sufficient.

On 7 December 2005, the Spanish police finally arrested Ante Gotovina with the help of the Croatian government on the Spanish island of Tenerife, located in the Canary Islands. He was brought to The Hague to be tried for war crimes. With the arrest of Ante Gotovina this issue was resolved, and entry negotiations began anew, after the certification of ICTY chief prosecutor Carla Del Ponte that Croatia now fully cooperated with the ICTY.

In April 2011, Croatian generals Gotovina and Markač were given extended prison sentences at the ICTY, which was widely perceived as unjust in the Croatian public. This caused a marked increase in opposition to the accession because, in the past, some countries like the Netherlands, conditioned their support for the Croatia's accession process with the continued Croatian co-operation with the ICTY.