Abstract The objective of this article is to account for the varying, and sometimes puzzling, outcomes of the past three Treaty revisions of EU/EC visa, asylum and immigration policy. The article focuses on decision rules and the institutional set-up of these policies, subjecting the results of the Intergovernmental Conference negotiations leading to the Treaties of Amsterdam and Nice and the Constitutional Treaty to causal analysis. The article maintains that four factors can explain the various Treaty outcomes: (i) functional pressures; (ii) the role of supranational institutions; (iii) socialization, deliberation and learning processes; and (iv) countervailing forces.