The term 'hunger' has a variety of meanings, and numerous physiological mechanisms underlying the sensation have been proposed. This review delineates the most common uses of the term, examines methods for measuring features of the experience and critically evaluates the evidence supporting its physiological basis. In addition, the association between reported hunger and ingestive behavior in healthy and selected clinical populations is discussed. Hunger is an important psychological and physiological phenomena that exerts a strong influence on behavior. When better characterized, it should be more amenable to monitoring and manipulation for therapeutic purposes.