Governance includes nonstate actors and nonlegal instruments. Nonstate governance – that which is developed, implemented, and/or enforced by nonstate actors – can fill roles that state law cannot or does so poorly. This chapter considers the extent to which nonstate actors do, could, and should contribute to solar geoengineering governance. It introduces key concepts of nonstate governance. The extant governance that is specific to solar geoengineering is largely nonstate. This is particularly evident in the development, influence, and apparent compliance with multiple sets of principles, such as the Oxford principles. Notably, for the most part, these sets substantively agree. The chapter closes with an analysis of nonstate governance’s potential. It concludes that nonstate governance should contribute because solar geoengineering’s characteristics – such as technically complexity, dynamism, reliance upon experts’ knowledge, transboundary impacts, and researchers' shared yet undifferentiated reputational sensitivity – are favorable to nonstate governance, while states are taking no significant steps toward governance.