XML (Extensible Markup Language) hasemerged as the standard data-exchangeformat for Internet-based businessapplications. These applications introduce anew set of data management requirementsinvolving XML. However, for the foreseeablefuture, a significant amount of business datawill continue to be stored in relationaldatabase systems. Thus, a bridge is neededto satisfy the requirements of these new XML-based applications while still using relationaldatabase technology. This paper describesthe design and implementation of the XTABLESmiddleware system, which we believeachieves this goal. In particular, XTABLESprovides a general framework to create XMLviews of relational data, query XML views,and store and query XML documents using arelational database system. Some of the novelfeatures of the XTABLES architecture are thatit (1) provides users with a single XML querylanguage for creating and querying XMLviews of relational data, (2) executes queriesefficiently by pushing most computation downto the relational database engine, (3) allowsusers to query seamlessly over relational dataand meta-data, and (4) allows users to writequeries that span XML documents and XMLviews of relational data.