The thesis focuses on core collective labour law rights and institutions as integral parts of the EU legal order, approached through the hermeneutical lens of the Union’s suggested ‘substantive constitutionalisation’. It seeks to establish labour law in general as an integral part of not only the internal market but the EU’s constitutional architecture as such, and collective labour rights in particular as a means to realise the Union’s socioeconomic objectives and an impetus for its democratisation. First, the thesis discusses the concept of constitutionalisation, its roots and various theoretical formats, and argues that, following the Lisbon Treaty, the EU has reached a point where it can be examined through the tools provided by its own ‘substantive constitution’. Fundamental values and objectives that form the normative foundation of the Union's constitutional architecture are examined, to reveal a mutli-layered and pluralist coherent framework. Within it, it is argued, labour law and collective labour institutions are to be placed and their role and function reconsidered accordingly. Second, focus turns on the substantive economic constitution of the Union and on collective labour institutions. The origins, nature, and essence of the ‘social market economy’ objective that sits at its core are explored, and a holistic understanding of the role and function of collective labour law and collective autonomy within it and as integral elements of a similarly holistic construction of the (aspired) political and the (existing) socioeconomic aspects of the EU is proposed. Consequently, the relevant jurisprudence of the CJEU, including case law related to EMU measures and mechanisms and euro crisis-induced interventions, is critically approached. The thesis ultimately aims to establish two propositions: first, that collective labour institutions and mechanisms are not antithetical to economic freedoms, but an integral condition for their effective exercise; second, that they are (constitutionally protected) institutions that serve the realisation of the multi-faceted normative goals, principles and aspirations of the EU, in pursuit of not just a common economic sphere, but more importantly, the establishment and development of a democratic and equitable polity, promoting participation and, thus, democratisation of the market and of EU constitutional perceptions.