Since the publication of Codd's paper, the relational databases have dominated the database world and are still in wide use nowadays. With the advance of the Web technologies, the database research community has oriented its focus on bridging the gap between the traditional ways of storing data using relational databases and the novel techniques of transferring data on the Web. XML has emerged as a standard for data transmission on the Web, by setting its main goal to be providing a simple and efficient way of storing and transferring data. Problems such as data integration, data exchange and answering queries using views have become a topic of interest in recent years, and their formalization in an XML setting has received a significant amount of attention. In this thesis, we propose a unified framework for analyzing and comparing the features of a multitude of works that analyze these problems in the context of XML. We introduce a query language for XML trees, called extended tree patterns, which allows us to define XML mapping assertions that successfully capture the expressive power of the mapping assertions used in a large subset of the works that we overview. We classify different approaches based on the expressive power of their mapping assertions and point out the similarities and differences along several criteria. Finally, we give an overview of the problems that have been addressed so far, and identify which specific variants of the respective problems have not been tackled yet.