Ontology reuse is turning into an important research issue in the ontology field. Ontology reuse can be seen from two points of view: (1) assembling, extending, specializing, adapting other ontologies which are parts of the resulting ontology, or (2) merging different ontologies on the same similar subject into a single one that unifies all of them. The first kind of reuse is named integration and is the central issue of this paper. In this article, we characterize the integration process, describe and discuss the activities that compose this process and propose an integration methodology. This integration methodology has successfully been applied to build two ontologies in different domains by reusing publicly available ontologies.