This chapter develops a conceptual framework for analyzing climate leadership among subnational governments in the Under2 Coalition. Drawing on multilevel governance theory, particularly Type II governance structures, the analysis positions subnational actors as autonomous entities operating within polycentric networks rather than hierarchical systems. The framework adopts Liefferink and Wurzel’s typology of climate leadership—cognitive, entrepreneurial, structural, and exemplary—while acknowledging empirical challenges in distinguishing between types. Central to the framework is reputation theory, which explains differential climate policy effort based on membership timing. Two core propositions guide the empirical analysis: first, founding members demonstrate higher climate policy effort than early and later joiners due to stronger reputational incentives; second, founding members exhibit greater policy durability over time through reputational lock-in effects that increase the costs of policy retreat. The chapter alludes to the importance of both strengthening factors (crisis events, technological developments, electoral dynamics) and weakening factors (economic pressures, political transitions) that shape climate leadership trajectories across temporal and political contexts.