Good neighbourliness is one of the most important principles relating to harmonious interstate relations. It primarily developed in international law around the idea of territorial sovereignty of states. The principle was further translated into an important accession condition in EU enlarge-ment policy. A violation of the good neighbourliness principle can lead to serious confrontations or military conflicts between states. Yet, the respect of the principle requires precise definition of its legal substance. As a paradox, the good neighbourliness principle has not been codified in international law. The lack of sufficient clarification of the essence of the principle potentially undermines the success of the Union’s engagement with it, threatening to lead to inconsistent interpretation and even to wrongful implementation of the good neighbourliness principle.