This chapter sets out what is “new” in the politics of numbers and this volume’s approach to their study. Rather than asking what quantification is, this volume is interested in describing and analysing what quantification does, tracking and unpacking various quantification practices and their manifold consequences in different domains. The book revisits the power of numbers, and examines changing relations between numbers and democracy. It engages, for the first time, Foucault inspired studies of quantification and the economics of convention in a critical dialogue. In so doing, the volume seeks to account more systematically for the plurality of the possible ways in which numbers can come to govern, highlighting not only disciplinary effects but also the collective mobilization capacities quantification can offer.