Abstract
This study tackles the primary problems facing sustainable tourism: the absence of any defined limits on sustainability in this sector and the difficulty of measuring sustainability. Based on a system of indicators calculated in municipalities that are representative of the different tourist areas and environments of Catalonia (Spain), the key variables in the study of sustainable local tourism are identified so that they can subsequently be normalised, weighted and aggregated in a single global score: the ISOST index. This index enables the definition of thresholds of sustainable tourism, thereby establishing a destination's level of sustainable tourism. It is a tool that can be applied to the definition of sustainable tourism strategies for the future.