This chapter describes the development of the Delphi method by the RAND Corporation in the 1950s, early criticisms of the method, and its subsequent evolution and growth in use. A wide range of variants of the Delphi method have been developed, only some of which aim to establish a consensus. The chapter examines contemporary uses of the method to establish consensus, covering its uses in making judgements on facts where the evidence is imperfect, setting methodological standards, making predictions, defining foundational concepts, determining collective values, improving professional practice, and improving policy.