There is considerable ambiguity about what makes an event a mega-event. Intervening in this debate, this paper develops a definition and classification scheme for mega-events. On the basis of a review of existing definitions, it proposes four constitutive dimensions of mega-events: visitor attractiveness, mediated reach, costs and transformative impact. The paper develops indicators for each dimension and maps onto these four dimensions a sample of the latest editions of nine large events (Expo, Summer and Winter Olympics, Football World Cup, European Football Championship, Asian Games, Commonwealth Games, Pan American Games, Universiade). From this, it develops a multi-dimensional, point-based classification scheme of large events according to size, distinguishing between major events, mega-events and the recently emerging class of giga-events. Concluding, it identifies the need for more systematic data on the size, costs and impacts of a broad range of large-scale events over time.