Why do some cities join transnational climate change networks while others do not? This study examines the factors that drive cities' participation in transnational climate change networks, such as the C40 Cities Climate Leadership Group and the Cities for Climate Protection program. Hierarchical analysis of 256 cities in 118 countries suggests that the degree of cities' globalization, or their level of “global cityness,” is positively associated with the cities' membership in the global networks. The level of individual cities' integration into the international economy and transportation grid is crucial for sharing ideas of global environmental responsibility. This tendency is found both in global cities of both developing and developed countries. Hierarchical models also suggest that attributes of cities—not country attributes such as democracy, income level, and being an Annex I country under the Kyoto Protocol—account for cities' memberships in transnational networks.