Research on NGO legal mobilization and judicial interventions have tended to focus on their activity before supranational and the highest national courts, especially the European courts and national supreme courts. However, while focusing our attention on these courts might be a shortcut to understanding the leading judgments shaping the law today, in so doing we run the risk of overrepresenting their role in the judicial system. Indeed, around the world, domestic courts working at subnational levels make frequent interventions in disputes. This is particularly the case in politically decentralized states where subnational entities have been assigned powers to pass legislation and elaborate their own policy. Spain serves as our case study here because it illustrates how a multilevel system of governance is pulled, by delegation of power, in the direction of both the supranational (EU) and regional authorities. We show that in Spain, the fact that the EU has led the way in such legislation, while the regions have not been so active, creates an opportunity for the Europeanization of law for environmental advocates. This chapter thus examines the patterns of environmental legal mobilization of NGOs at the subnational level in Spain across regional high courts.