Substantial and increasing amounts of funding are available for countries to undertake climate change interventions. This article argues that to ensure effective allocation of these resources, the selection and design of climate change mitigation and adaptation interventions should be based on evidence of what works, what doesn't work, under what circumstances and at what cost. Currently the evidence base on the impact of climate change interventions is minimal and there is a need for wider application of rigorous impact evaluation (IE) in the field. Climate change interventions have much to learn from experiences in related fields, notably international development and conservation. The paper highlights some of the challenges faced when conducting IEs of climate change interventions and discusses how these can be tackled. Moreover, it discusses some of the key areas of mitigation and adaptation interventions and suggests how IEs could be implemented, using IEs from other policy fields as examples. It argues that despite the limited experience so far there are ample opportunities to conduct IE of climate change interventions. If calls for increasing financing of climate change mitigation and adaptation by hundreds of billions of dollars a year are to remain credible and gain support, evidence of the effectiveness of current spending is essential.