The increasing amount of structured data available on the Web is laying the foundations for a global-scale knowledge base. But the ever increasing amount of Semantic Web data gives rise to the question – how complete is that data? Though data on the Semantic Web is generally incomplete, some may indeed be complete. In this book, the author deals with how to manage and consume completeness information about Semantic Web data. In particular, the book explores how completeness information can guarantee the completeness of query answering. Optimization techniques for completeness reasoning and the conducting of experimental evaluations are provided to show the feasibility of the approaches, as well as a technique for checking the soundness of queries with negation via reduction to query completeness checking. Other topics covered include completeness information with timestamps, and two demonstrators – CORNER and COOL-WD – are provided to show how a completeness framework can be realized. Finally, the book investigates an automated method to generate completeness statements from text on the Web. The book will be of interest to anyone whose work involves dealing with Web-data completeness.