The one-university idea--that the University of California is a single institution whose campuses are united in the pursuit of a common mission and common standards of quality--has been a guiding organizational principle since UC President Robert Gordon Sproul first articulated it in the 1930s. This paper examines the origins of the one-university idea in the Sproul era, the role it has played in UC's institutional development through waves of decentralization and campus expansion, and whether it remains relevant today. (Contains 38 footnotes.)