In this dissertation, we study serving of the user's sophisticated ad hoc information needs based on beforehand unknown, autonomous and heterogeneous XML data sources. Serving sophisticated information needs that concern comparing and analyzing products, places, individuals, and so on, often requires that the underlying data are combined from disparate and independently organized data sources. In other words, data integration is needed. However, when the information needs at hand are ad hoc or short-term in nature, the use of traditional data integration techniques is often not an option because of the time and resources their effective application requires. Moreover, they also typically presume considerable technical expertise from their user as well as intimate familiarity with the underlying data sources.