This article examines the role of ambiguity in processes of institutional change. One challenge for understanding institutional change is to overcome the rather "oversocialized" view of action within Institutional theory. Drawing upon recent work in sociology, the paper introduces a non-teleological model of action that stresses the ambiguity of institutionalized beliefs. Ambiguity is then applied to Masahiko Aoki's concept of institutions as "summary representation" of a strategic game. Rather than institutional break down, ambiguity is associated with incremental modes of institutional change through creative reinterpretation and redeployment of old institutions for new purposes. Empirically, the paper applies these considerations to understanding the historical evolution of employee codetermination in Germany. The continuity in formal legal rules of codetermination contrasts with remarkable diversity as an organizational practice-over time, across industrial sectors and between individual firms. Codetermination illustrates how ambiguity originated in political compromise, but also how ambiguous agreement allows scope for institutional innovation. Ambiguity is, thus, central for understanding how codetermination was partially reproduced and partially changed over time.