This chapter introduces the notion of data activism as a heuristic tool that allows investigating how big data affect contemporary society without dismissing the democratic agency of citizens. It explores data activism as the newest form of media activism and puts forward the notion of data activism as a heuristic tool able to bring democratic agency back into the analysis of how big data affect contemporary society. The chapter straddles three streams of literature, namely media studies, social movement studies, and science and technology studies (STS), in view of offering an all-round theoretical and empirical approach to investigate people's critical engagement with massive data collection. It distinguishes between two forms of data activism, namely proactive and reactive. The chapter further provides historical grounding to the concept, placing data activism within the contemporary movement ecology and analyzing its role in relation to technological innovation and to media activism in its historical and current interpretations.