How has the transformation of treaty-making rules and norms affected the practice of EU treaty making over time? Using quantitative methods, this chapter shows that the rise of parliaments and referendums is associated with declining rates of treaty making over time. Faced with tighter ratification constraints, national governments have, it is noted, sought to untie their hands by rerunning unfavorable referendums and pursuing international law treaties negotiated and ratified at one remove from EU constitutional rules and norms. And yet the EU’s frequent recourse to treaty amendment in the 1990s and 2000s can be viewed as an attempt to address the problems of legitimacy exposed by earlier rounds of treaty making.