This working paper examines the phenomenon of social exclusion, identifying the different factors at work and the relative importance of each. The pages that follow set out the results of a study pursuing these objectives. We look at how far social exclusion can serve to explain the emerging processes of social fragmentation, then go on to propose a system for approaching the phenomenon in quantitative terms, comprising a set of exclusion indicators based on the identification of all the factors intervening from different, though closely interrelated, exclusion spheres.