This article presents a methodology for the operational definition, quantification and characterization of day trippers, for use at the local or regional levels. The methodology stresses the importance of such concepts as 'daily urban systems', 'functional areas', 'travel-to-work areas' and other similar aggregations to define what is one of the main features of tourism: 'usual environment'. Different systems are developed for the quantification of day trippers, based on both primary (fieldwork) and secondary data, and we apply both to the case of a comarca in the Province of Barcelona (Catalonia). The results show the relevance of the phenomenon of 'same-day trips' (for tourism) and the interest for defining and characterizing this phenomenon correctly in order to implement tourism policies that address the different profiles presented by day trippers.