In this paper we present the current version of Mastro, a system for ontologybased data access (OBDA) developed at Sapienza Università di Roma. Mastro allows users for accessing external data sources by querying an ontology expressed in a fragment of the W3C Web Ontology Language (OWL). As in data integration [5], mappings are used in OBDA to specify the correspondence between a unified view of the domain (called global schema in data integration terminology) and the data stored at the sources. The distinguishing feature of the OBDA approach, however, is the fact that the global schema is specified using an ontology language, which typically allows to provide a rich conceptualization of the domain of interest, independently from the source representation
