Governments are increasingly turning to public sector innovation (PSI) labs to take new approaches to policy and service design. This turn towards PSI labs, which has accelerated in more recent years, has been linked to a number of trends. These include growing interest in evidence-based policymaking and the application of ‘design thinking’ to policymaking, although these trends sit uncomfortably together. According to their proponents, PSI labs are helping to create a new era of experimental government and rapid experimentation in policy design. But what do these PSI labs do? How do they differ from other public sector change agents and policy actors? What approaches do they bring to addressing contemporary policymaking? And how do they relate to other developments in policy design such as the growing interest in evidence-based policy and design experiments? The rise of PSI labs has thus far received little attention from policy scientists. Focusing on the problems associated with conceptualising PSI labs and clearly situating them in the policy process, this paper provides an analysis of some of the most prominent PSI labs. It examines whether labs can be classified into distinct types, their relationship to government and other policy actors and the principal methodological practices and commitments underpinning their approach to policymaking. Throughout, the paper considers how the rise of PSI labs may challenge positivist framings of policymaking as an empirically driven decision process.