In this paper, critical realism and activity theory are compared within the context of theorizing technology-mediated organizational change. An activity theoretic analysis of the implementation of large-scale disruptive information systems in a public sector setting (in particular concerning paramedic treatment of heart attack patients and ambulance dispatch work activity) is used to illustrate how activity theory makes a significant contribution to critical realism, by (1) locating technology within "activity systems" and theorizing change through contradictions and congruencies within those systems; (2) developing recent critical realism-inspired theorization of the "inscription" of cultural and social relations within technology; and (3) developing recent insights of critical realist researchers regarding the way in which the performance management agenda is mediated through IS.