Much of the attention surrounding local climate protection in the United States is associated with two networks: ICLEI Local Governments for Sustainability's Cities for Climate Protection and the U.S. Conference of Mayors’ Climate Protection Agreement (MCPA). However, the impact of these networks on member-city actions has not been clearly established. This paper examines whether, and to what extent, participation in climate networks leads to the implementation of greenhouse gas (GHG)-reducing policies, above and beyond what would have been done in their absence. To account for the possibility that cities which join climate networks are systematically different from those that do not and control for self-selection induced bias, three statistical techniques—propensity score matching, Heckman full information maximum likelihood, and instrumental variables—are employed to estimate the “treatment effect” of participation. Results suggest that impact is network specific: ICLEI membership causes small to moderate increases in cities’ GHG-relevant activity, whereas the MCPA has no such effect. The characteristics of each network are evaluated in light of these findings.