XML is emerging as the universal standard format for data exchange, and as a result, XML data management is becoming increasingly important. Relational DataBase Management Systems (RDBMSs) have dominated the commercial data management space for several decades. So, using RDBMSs to manage XML data is an attractive option. Unfortunately, the XML data model differs substantially from the relational data model, so using RDBMSs to support XML data poses a number of interesting and challenging problems. The need for interoperation and data exchange through the Internet has made eXtensible Markup Language (XML) a dominant standard language.