Climate change governance is becoming more polycentric. From the global to the municipal level, an unprecedented number of actors are addressing climate change. But will polycentric governance generate significant emissions reductions? What brings disperse action together to build cumulative outcomes capable of limiting climate change to well below two degrees of warming? How will actors in different institutions and jurisdictions be able to learn from one another and thereby generate significant governance synergies? These important questions relate directly to the vexed question of policy evaluation, an underappreciated, but potentially highly consequential element of polycentric climate governance. In spite of substantial growth of evaluation activity and theorization, its role in assessing and possibly improving climate governance has been insufficiently explored. This chapter introduces the book, which tackles this challenge head-on by developing new, cutting edge theory and then conducting an empirical test in the context of the European Union, Germany and the United Kingdom.