This study investigates the phenomenon of polarity sensitivity, proposing a new perspective which focuses on the behavior and properties of sensitive items and the phrases they form. It rests on the observation that the complexity of the phenomenon requires a more articulated analysis than the standard one based on licensing conditions. The study adopts a broader notion of sensitivity, which extends beyond the traditional one restricted to polarity. It includes other factors which are defined in relation to the semantics of the sensitive items. As a result, the attention is centered on the web of interactions the item entertains with its context of occurrence, rather than on polarity licensing conditions. These items are not lexical black boxes, mutually undistinguishable with respect to polarity licensing. The study shows that the distribution and interpretation of these items follow from constraints related to the properties of their semantic domain. Aspect and temporal order are crucial factors for sensitive temporal connectives and adverbials, while referential links and existential import matter for determiners. Analogies and differences with other phenomena related to negation, e.g. negative concord, are discussed. The volume also contains an extensive critical overview of research on the topic in the past thirty years. This book will be of interest to scholars in the area of semantics, syntax and their interface. It also points to various important themes in pragmatics and cognitive science, e.g. the role of context and reference, and the notion of interpretation strategies.