Cities have become crucial actors for the global governance of climate change. Their increased activity in this field is reflected by the rising number of adoptions of local climate strategies in an original sample of 274 European cities from 1992 to 2009. Using event history analysis, I find that this spread is promoted by transnational municipal networks (TMNs) successfully deploying strategies for governance by diffusion, their impact exceeding that of most alternative explanatory factors cited in the literature. Given their capacity to foster the spread of climate policy innovations among cities, TMNs can thus be expected to play a decisive role in a climate governance system that is becoming increasingly fragmented, polycentric, and transnational.