We investigate the use of epistemic operators in the framework of concept languages (also called terminological languages). The results of this work have a twofold signiicance. From the point of view of epistemic logics, our contribution is to have identiied an effective procedure for the problem of answering epistemic queries posed to a knowledge base expressed in the concept language ALC. From the point of view of concept languages, the most relevant aspect of our work is that we have reconstructed in logic several common features of existing knowledge representation systems. Epistemic operators provide a highly expressive query language; allow for the treatment of several database features, such as closed world reasoning and integrity constraints; and nally can give a formal characterization of some procedural mechanisms , such as trigger rules, usually considered in frame-based systems.