What are the epistemological and political contours of evidence today? This introduction to the special issue lays out key shifts in the contemporary politics of knowledge and describes the collective contribution of the six papers as an articulation of what we describe as a ‘new empiricism’, exploring how earlier historical appeals to evidence to defend political power and decision-making both chime with and differ from those of the contemporary era. We outline some emerging empirical frontiers in the study of instruments of calculation, from the evolution of the randomized controlled trial (RCT) to the growing importance of big data, and explore how these methodological transformations intersect with the alleged crisis of expertise in the ‘post-truth’ era. In so doing, we suggest that the ambiguity of evidence can be a powerful tool in itself, and we relate this ambiguity to the ideological commitment and moral fervour that is elicited through appeals to, and the performance of, evaluation.