This chapter introduces a method re-using a thesaurus built for a given domain, in order to create new resources of a higher semantic level in the form of an ontology. Considering ontologies for data-mining tasks relies on the intuition that the meaning of textual information depends on the conceptual relations between the objects to which they refer rather than on the linguistic and statistical relations of their content. To put forward such advanced mechanisms, the first step is to build the ontologies. The originality of the method is that it is based both on the knowledge extracted from a thesaurus and on the knowledge semi-automatically extracted from a textual corpus. The whole process is semi-automated and experts’ tasks are limited to validating certain steps. In parallel, we have developed mechanisms based on the obtained ontology to accomplish a science monitoring task. An example will be given.