This chapter examines the involvement of Spanish local governments in transnational networks of municipalities (TMNs) within climate and energy policies, as part of the global multilevel governance system. The research is descriptive and based on a theoretical framework of multilevel governance, environmental governance, and public policy networks. It aims to identify the main TMNs in which Spanish local authorities are involved; characterize these networks by size, complexity, and member profiles; explore their functioning; and analyze the participation of Spanish local authorities by territory and its evolution. The study applies a qualitative methodology, analyzing information from various bibliographic and documentary sources, and focusing on ten TMNs of both European and global scope. Quantitative data are used to illustrate the most distinctive features of local governments, and to assist in the description of the analyzed networks and the presence subnational governments. Spanish local governments participate in several of the main existing TMNs. However, the presence of local governments in the TMNs is much greater in some networks than in others, and varies depending on the characteristics of the local governments themselves and the Autonomous Community considered. The presence of local authorities above the municipal level often facilitates the involvement of municipalities in TMNs.