Nowadays, the interest to work simultaneously with data stored in several databases is growing. Multidatabase Systems (MDBS) have been proposed as a solution to work with different pre-existing autonomous databases. Federated Database Systems (FDBS) are a special type of MDBS where an integrated schema is provided. This integrated schema is the result of an integration process among the schemata of the pre-existing autonomous databases. In our case we have built a FDBS that integrates several heterogeneous relational databases by using a particular type of Knowledge Representation system based on Description Logics (DL system). The integrated schema is represented as a terminology formed by a set of classes and attributes. Although there has been a lot of research about the problems of translation and integration of schemata to obtain integrated ones, the problem of query processing against these integrated schemata has not been treated so much. In this paper we present an overview of the query processing of our FDBS and then we point out the features of DL systems used in the query processing.