This introductory article situates the study of gratitude within the historiography on emotions and transatlantic relations, particularly in relation to the affective turn in the humanities and social sciences. It also sets out the common goals of the articles: inviting historians to take gratitude seriously by defining its relationship to ongoing work in memory, humanitarian and international studies. By identifying a diplomacy of gratitude, it centres on emotional practices and performances and illustrates how this draws in new actors, behaviours and processes to our overall understanding of European–United States relations. Finally, it argues how the contributions to this special issue collectively reveal the historical development of repertoires and processes of gratitude throughout the ‘Transatlantic Century’.1