The Internet and the World Wide Web are becoming increasingly important in our highly interconnected world. This book addresses the topic of querying the data available, with regard to its quality, in a systematic and comprehensive way, from a database point of view. First, information quality and information quality measures are systematically introduced before ranking algorithms are developed for selecting Web sources for access. The second part is devoted to quality-driven query answering, particularly to query planning methods and algorithms. The in-depth presentation of algorithms and techniques for quality-oriented querying will serve as a valuable source of reference for R&D professionals and for IT business people. In addition, the work will provide students with a comprehensible introduction to cutting-edge research.