FB͑<\?sPw~o ٔ\=KHa>nDQ#h׊zpzwN?)bYwO5 Wf:6GX [[Z6mFE5c'Y9N3FYV2xZy7@I:ݏ.R?N"VTyhԋ1 mV!3bKc3r mentation of software. In this talk, I will argue that variants of the notion of a conservative extension should play an even more important role in ontology engineering. Firstly, they can be used to define the notion of a refinement of an ontology. Secondly, they can form a uniform basis for introducing the notion of a module of an ontology and modularised ontologies. And finally, first results indicate the feasibility of reasoning support for conservative extensions in ontology languages based on description logics such as OWL-DL. !""""