This book clarifies all legal aspects of the good neighbourliness principle in international and EU law. It elaborates on the most acute infringements of the principle of good neighbourliness in both international relations and EU enlargement policy. Taking a two-part approach, it offers the international law and EU law perspectives. Over five chapters, it sets out the legal framework and the practical application of the principle, before setting out conclusions on the effectiveness of that application. Given the current focus on enlargement and accession to the EU this is a timely and topical study in an under-researched field.