 Although not all philosophical references to preference make use of formal tools, preferences are almost always assumed to have structural properties of a type that is best described in a formalized language. The study of the structural properties of preferences can be traced back to Book III of Aristotle’s Topics. Since the early twentieth century several philosophers have studied the structure of preferences with logical tools. In 1957 and in 1963, respectively, Sören Halldén and Georg Henrik von Wright proposed the first complete systems of preference logic (Halldén 1957, von Wright 1963). The subject also has important roots in utility theory and in the theory of games and decisions. The preferences studied in preference logic are usually the preferences of rational individuals, but preference logic is also used in psychology and behavioural economics, where the emphasis is on actual preferences as revealed in behaviour.