Why and under which conditions do international student assessment programmes like PISA have success? How can the results of these assessments be useful for advocates of different, even contradictory, policies? What might explain different patterns of using assessment as a tool for school governance? Drawing on historical and comparative research, and using PISA as an example, this paper provides a frame for discussing these and other questions around the international rise of accountability as a key tool of social change. The basic argument is that even though accountability is a global phenomenon, the ways and means of enacting and encountering accountability are not. How accountability is experienced depends on deeply engrained 'constitutional mind-sets', i.e. diverse cultures of conceptualizing the relation between the public and its institutions.