Informal dimensions of European integration have received limited academic attention to date, despite their historical and contemporary importance. Particularly studies in European integration history, while frequently mentioning informal processes, have as yet rarely conceptualised the study of informality in European integration, and thus fail usually to systematically analyse conditions, impact and consequences of informal action. This research project, which I coordinated together with Lennaert van Heumen (Radboud University Nijmegen), shows through both successful and failed examples of informal action how informality has impacted the functioning of the European Communities and later the EU, as well as other European and transatlantic organisations, such as the WEU, the G7 and NATO. In addition to specific case studies which we assembled, we have developed a concise research guide to the study of informality. Reflecting the diversity of studies within this burgeoning field of research, we brought together scholars approaching the informal dimensions of European integration from different disciplinary, methodological and thematic angles.