The next section discusses the issue of delimitating the core concept of the present volume, i.e. participation. A second section then explains how an outcomecentric research design is useful for addressing the first and the second questions of this volume (i.e. how and why the EU participates in the long term). A third section details how a factor-centric research design is relevant for dealing with the third question raised (i.e. the effects of long-term participation). The conclusion proposes to go one step further and, by building on this publication and on other future researches, to engage in quantitative analysis of the participation of the EU in different IOs.