Relevance drives our actions and channels our attention; it shapes how we make sense of the world and communicate with each other. Irrelevance spreads a twilight which blurs the line between information we do not want to access and information we cannot access. In disciplines as diverse as philosophy, sociology, the information sciences and linguistics, "relevance" has been proposed as a key concept. This book is the first to bring together the often unrelated traditions. Researchers from different fields discuss relevance and relate it to the challenges of "irrelevance", which have so far been neglected despite their significance for our chances of making well-informed decisions and understanding others. The contributions focus on theoretical and conceptual questions, on specific factors and fields, and on practical and political implications of relevance and irrelevance as forces which are even stronger when they remain in the background.