The Centre de Données Astronomiques de Strasbourg (CDS) will be a major hub of the emerging Virtual Observatory. It provides reference services such as SIMBAD, VizieR and Aladin, which are essential for data interpretation and are used for critical functionalities by other services (e.g. SIMBAD as name resolver for the ADS and the major on-line observatory archives). Each service can be used as an entry point towards complementary information systems: archived data and images, results published in journals, etc... SIMBAD integrates reference information about objects. Catalogs, tables published in journals, observation logs and surveys are integrated in the VizieR service. The Aladin interactive sky atlas goes further towards integration of distributed image databases. The cross-correlation tool built in the frame of the ESO/CDS Data Mining Project, in prototype phase, addresses an important concern of the Virtual Observatory. CDS will also provide generic tools (e.g. the GLU). It plays a rôle in the definition of metadata and exchange standards, in particular in the context of XML: the Astrores XML description of tabular data has been defined in a large collaboration, and Aladin, which is fully XML compatible, can be seen as a prototype of XML-built tool to integrate heterogeneous data.