The majority of anthropogenic greenhouse gas emissions result from the activities of non-state actors (NSAs). States recognize the need to engage with NSAs as mitigation actors, including by encouraging or requiring them to pledge, or commit to, mitigation action. NSAs are also making waves through civil-society organizations (CSOs) bringing cases to court to test the legal obligations of large corporations. This chapter reflects on the academic and social debates on whether NSAs—in particular, companies, cities, and CSOs—have any legal obligation to mitigate climate change or could meaningfully assume such an obligation, and whether they have demonstrated any kind of effective leadership in mitigation action. The author finds that, while NSAs assert themselves as high-profile players in the mitigation realm, their effectiveness is unclear, and so is their theorization as actors from a legal-scholarly perspective.