In parallel with the rapid growth of the World Wide Web, arose the need for a common data format. Having proprietary data formats was sufficient for managing data within businesses or even for communicating data across a small number of partners; but this model was not scalable. The advantages of interoperability, exemplified by the Web, gave rise to the need for a highly standardized common data format for data exchange between applications. The solution to this problem came with the advent of XML. Given the increasing number of applications that use XML to exchange, mediate, and store data, tools for effective management of XML data are becoming increasingly important. In particular, tools for querying and transformation of XML data are essential to extract information from large bodies of XML data, and to convert data between different representations (schemas) in XML. Just as the output of a relational query is a relation, the output of an XML query can be an XML document. This paper tries to give a closer overview of the state of the query processing in the context of XML database systems.