From Philosophy, ontology is “the science of what is, of the kinds and structures of objects, properties, events, processes and relations”. In computer sciences and knowledge management an “ontology” is a specification of a conceptualization of a given knowledge domain. For multi-agent simulation, the domain is models rather than data. To answer the question “Why an ontological framework for the multi-agent modelling in the Social Sciences?”, this paper deals first with three dimensions: (1) model engineering, (2) thematical and epistemological issues and (3) model assessment and comparisons (ontological test). Contrary to several ontologies, this paper does not propose a single representation of the knowledge domain, but a possible plurality, based on the concept of “knowledge framework” building to integrate the plurality of “point of view” co-existing in the Social Sciences within a general framework. Accordingly, the last part presents some examples of ontological points of view on a model of residential segregation derived from the Schelling’s one.