Polycentric climate governance holds enormous promise, but to unleash its full force, policy evaluation needs a stronger role in it. This book develops Nobel Laureate Elinor Ostrom's important work by offering fresh perspectives from cutting-edge thinking on climate governance and policy evaluation. Driven by theoretical innovation and empirical exploration, this book not only argues for a stronger connection between polycentric climate governance and practices of evaluation, but also demonstrates the key value of doing so with a real-world, empirical test in the polycentric setting of the European Union. This book offers a crucial step to take climate governance to the next level. It will be of interest to advanced students and researchers in climate governance, as well as practitioners who seek to enhance climate action, which is needed to avoid a climate catastrophe and to identify a pathway towards the 1.5° Celsius target in the Paris Agreement.