In this book, we propose to treat emotion as the principal factor in the system of needs of a searcher, and therefore one that ought to be considered by the information retrieval systems. The main objective of Information Retrieval (IR) systems is to satisfy searchers' needs. We present a more realistic view of searchers' needs by considering not only theories from information retrieval and information science, but also from psychology, philosophy, and sociology. We extensively report on the role of emotion in every aspect of human behaviour, both at an individual and social level. This serves not only to modify the current IR views of emotion, but more importantly to uncover social situations where emotion is the primary factor (i.e., source of motivation) in an Information Retrieval and Seeking process. We also show that the emotion aspect of documents plays an important part in satisfying the searcher's need, in particular when emotion is indeed a primary factor. Given the above, we define three concepts, called emotion need, emotion object and emotion relevance, and present a conceptual map that utilises these concepts in IR tasks and scenarios.