The accession of ten new Member States to the EU on the 1st of May 2004 was but a culmination of far-reaching preparation efforts in the candidate countries, as well as complex multilateral negotiations, which resulted in the conclusion of the 2003 Accession Treaty, an elaborate legal instrument containing adaptations of EU law and transitional arrangements. Taking as a starting point the principle of immediate and full application of EU law in the new Member States, this chapter retraces and analyses some of the accession-related legal problems, which were addressed in the case law of the EU courts related to the interpretation of the 2003 Accession Treaty. These issues are grouped into two categories, concerning, respectively, the temporal and the material scope of EU law in the new Member States. The temporal issues raised in the post-accession litigation covered diverse matters such as the late publication of EU acts in the new Member States, the temporal scope of the Court’s preliminary rulings jurisdiction and the judicial protection of the new Member States in the interim period. The substantive legal issues are related to a wide range of EU law areas stemming from the fundamental freedoms of the internal market to the application of State aid law in the new Member States. The common thread of those cases is their link to horizontal jurisprudential developments in EU law. The analysis of judgments of EU courts concerning the interpretation of the 2003 Accession Treaty shows that legal problems, which at first sight may appear to concern isolated issues related to enlargement, have influenced the EU court’s case law on such fundamental aspects of EU law as the interpretation of the principle of legal certainty, the scope of fundamental market freedoms or the invocability of the Charter in infringement proceedings.