Brexit and the ensuing uncertainty about the United Kingdom's (UK's) future relationship with the European Union (EU) have brought the participation of non-EU Member States in the internal market to the centre of academic attention. The latter phenomenon is not novel and many of the existing frameworks for cooperation between the EU and its neighbourhood countries have been used as models for a possible post-Brexit arrangement. This article identifies the various roles played by the internal market acquis - both of integration and disintegration - in the EU's relations with its neighbourhood by analysing the dynamics between the aims of various bilateral and multilateral instruments and the character and scope of the internal market acquis contained therein. The article argues that over time the function of the internal market acquis has evolved from providing a legal framework for the functioning of the internal market among the EU's Member States to also integrating third countries into the Union's sphere of influence beyond the accession process, and even membership. The internal market can thus no longer be regarded as an 'internal' and exclusive affair for the committed few that offers inspiration and limited access for third countries but rather as a dynamic and geographically inclusive form of collaboration between the Union and its periphery.