Relational databases get more and more employed in order to store the content of a web site. At the same time, XML is fast emerging as the dominant standard at the hypertext level of web site management describing pages and links between them. Thus, the integration of XML with relational database systems to enable the storage, retrieval and update of XML documents is of major importance. This paper presents X-Ray, a generic approach for integrating XML with relational database systems. The key idea is that mappings may be defined between XML DTDs and relational schemata while preserving their autonomy. This is made possible by introducing a meta schema and meta knowledge for resolving data model heterogeneity and schema heterogeneity. Since the mapping knowledge is not hard-coded but rather reified within the meta schema, maintainability and changeability is enhanced. The meta schema provides the basis for X-Ray to automatically compose XML documents out of the relational database when requested and decompose them when they have to be stored.