Credibility in Information Retrieval presents a detailed analysis of existing credibility models from different information seeking research areas, with a focus on the Web and its pervasive social component. It shows that there is a very rich body of work pertaining to different aspects and interpretations of credibility, particularly for different types of textual content (for example, Web sites, blogs, tweets), but also for other modalities (such as videos, images, audio) and topics (such as health care). This book defines the limits of credibility with respect to digital information access systems, providing the reader with an organized and comprehensive reference guide to the state of the art and the problems at hand. As an additional help to the reader, an appendix lists the existing test collections that cater specifically to some aspect of credibility.