The past few years have seen a dramatic increase in the popularity and adoption of XML, the eXtensible Markup Language. This explosive growth is driven by its ability to provide a standardized, extensible means of including semantic information within documents describing semi-structured data. This makes it possible to address the shortcomings of existing markup languages such as HTML and support data exchange in e-business environments.Consider, for instance, the simple HTML document in Listing P.1. The data contained in the document is intertwined with information about its presentation. In fact, the tags describe only how the data is to be formatted. There is no semantic information that the data represents a person's name and address. Consequently, an interpreter cannot make any sound judgments about the semantics as the tags could as well have enclosed information about a car and its parts. Systems such as WIRE (Aggarwal et al. 1998) can interpret the information by using search templates based on the structure of HTML files and the importance of information enclosed in tags defining headings and so forth. However, such interpretation lacks soundness, and its accuracy is context dependent.