This project investigates a ubiquitous yet under-studied phenomenon in national, European and global politics: delegated preparation, defined as those negotiations through which civil servants or experts "pre-cook" political choice in multi-level decision-processes. While examples are legion-reaching from legislative drafting in national ministries to the Committee of Permanent Representatives (COREPER) in the European Union (EU)- the project focuses on preparation in complex international negotiations, and chooses EU Intergovernmental Conferences (IGCs) as empirical case. Claiming that a look beyond the tip of the "decision-iceberg" will gain us deeper insights into how and by whom Europe has been constitutionalised, I tackle two wider questions: 1) What is preparation and what can it do? and 2) Under what conditions will preparation be effective? Linked to an understanding of international negotiation as a "thick" social process, I argue, first, that the key to preparatory effectiveness lies in a particular set of collective resources as a necessary condition, and in consensual preagreement as both necessary and sufficient. Second, with effective pre-decision-making thus hinging upon successful delegated arguing, a set of scope conditions favourable to persuasion are singled out. These include 1) a familiar, iterative and insulated social context as a pre-condition for the non-distortive use of arguments; 2) an issue's complexity as facilitating the resonance of expertise and novel ideas; and 3) a macronorm's constitutional-systemic nature as favouring factual arguments linked to the international system. The hypotheses are tested on the "Group of Government Representatives" (GoR), with units of observation chosen from the Amsterdam and Nice IGCs according to variation of issue complexity and constitutional-systemic nature. Process-tracing of five issues: the communitarisation of free movement, the integration of Schengen and the institutionalisation of flexibility (Amsterdam), as well as Commission reform and Council votes (Nice) confirms that delegated preparation plays a key role even in the "bastion of high politics" that is EU reform. Yet, empirical evidence shows that persuasion is less prominent than expected, and uncovers alternative mechanisms behind effective preparation,in particular accommodation, depoliticisation and systemic compensation.