This book presents a theory of attention. According to this theory the relationship between attention and the processes executed in the brain is analogous to the relationship between unison and the processes executed by individual members of an orchestra: Just as no subset of the players in an orchestra can be identified as the ones responsible for unison, so there are no particular processes in the brain that are the implementers of attention. If this is right then attention belongs in the metaphysical category of 'adverbial phenomena', and so is not the sort of thing that can be explained by identifying the processes that constitute it. The book therefore provides a case study of the ways in which metaphysical questions and questions about psychological explanation can interact. It also explores the prospects of using the theory of attention to cast explanatory light on consciousness and on the contentfulness of thought.