The book examines the twofold ''boundaries'' of the concept of the European Union''s internal market – the geographical and the substantive – through the prism of expanding the internal market to third countries without enlarging the Union. The book offers a comprehensive analysis of the conditions under which the internal market can effectively be extended to third countries by exporting EU acquis via international agreements without sacrificing its defining characteristics. Theoretical rather than empirical in approach, the book scrutinises and meticulously questions the required level of uniformity within flexible integration relating to the substantive scope of the internal market, the role of foundational principles in the European Union''s market edifice, and the institutional framework necessary for granting third country actors full participation in the internal market while safeguarding the autonomy of the Union''s legal order.