Data integration is the process of facilitating the access to data residing at multiple heterogeneous data sources, and providing users and applications with a unified view of these data. Data integration is crucial in enterprises that store and exchange data in disparate and incompatible formats, and is one of the big challenges of the World Wide Web, where millions of heterogeneous data sources are available. This book contributes to different aspects of the design of modern data integration systems in the context of the Web. On one hand, the work contributes to the Semantic Integration research trend, which refers to the problem of reconciling data from autonomous sources using ontologies and other semantic-based tools. The work suggests a novel solution to XML-RDF semantic integration and also contributes to the problem of Ontology Alignment, defining a rigorous and scalable semantic similarity measure for RDF labelled directed graphs. On the other hand, the book suggests a novel solution to the problem of translating a user query (targeting a logical mediated schema), into queries over a set of autonomous data sources provided with restricted web interfaces.