Anno Bunnik explores the ramifications of Big Data for countering terrorism, extremism, and radicalisation. With the recent rise of jihadists and other extremists groups in the Middle East and Europe, how do state agencies respond through the use of Big Data? This chapter critically engages with questions such as the extent to which Big Data can inform us about and help prevent radicalisation. It deals with the role of non-governmental organisations and the private sector in this regard. Bunnik makes the argument that whilst it is inevitable that Big Data will take centre stage in counter-terrorism, its potential should also be utilised to better understand radicalisation and extremism.