Very roughly speaking, intentionality is the characteristic that a mental state has if it represents or is directed on an entity (where the entity may be a proposition or a state of affairs). Philosophers have found this characteristic to be elusive and confusing, and they have been increasingly concerned to explain it. One approach consists in attempting to explain intentionality by giving reductive accounts of such intentional concepts as belief and desire. This approach can take several different forms: in the past reductionists thought that it might be possible to provide reductive definitions of intentional concepts in terms of concepts that stand for forms of behavior, but more recently they have sought to give reductive definitions in terms of the concepts of a formal discipline like computing or information theory. A second approach consists in trying to make explicit the principles that underlie our use of intentional concepts in everyday life. Advocates of this approach maintain that the content of an intentional concept is determined by the role that it plays in these principles. They also maintain that philosophical questions about belief and its fellows can be answered by enumerating the principles and explaining their logical and semantic properties.