The purpose of this contribution is to discuss the extent of the Union's competence to adopt a comprehensive instrument of substantive contract law in light of the options set forth in the Commission's 2010 Green Paper on European contract law, with a view to exploring the limits of, and the relationship between, certain prominent Treaty provisions in the debate -- namely, Articles 114, 115, 81, 169 and 352 TFEU (ex Articles 95, 94, 65, 153 and 308 EC, respectively) -- and the roles played by the principles of subsidiarity and proportionality in guiding the exercise of Union competence inside, as well as outside, the EU decision-making process. Moreover, given the reference to certain American techniques in the Green Paper, this contribution seeks to facilitate discussion of the comparative dimensions of the constitutional assessment of contract law in the European Union and the United States. [ABSTRACT FROM AUTHOR]