Urgenda is an association, established under Dutch law, with the purpose of encouraging the acceleration of the transition towards a more sustainable society, beginning in the Netherlands. Urgenda persuaded a Dutch civil court to rule that, in order not to contribute to dangerous climate change, the Dutch State had to reduce greenhouse gas emissions in and from the Netherlands by at least 25 per cent in 2020 when compared with the levels in 1990. The court held that the provisions in the Climate Change Convention, the Kyoto Protocol, the no harm principle of customary international environmental law, and certain provisions of EU law, all contained inter-State commitments and/or were too vague to be directly invoked by Urgenda against the State. These norms could, however, be used to flesh out and give concrete meaning to open norms of Dutch domestic civil law, such as the duty of care. Articles 2 and 8 of the European Convention on Human Rights, which could not be invoked directly because Urgenda was not itself a victim of a breach of these provisions, served a similar function. The central question that will be addressed in this chapter is whether the Urgenda case can be seen as a successful example of public interest litigation for the protection of the environment. We speak of public interest litigation when associations like Urgenda, established under domestic law to pursue a certain general interest, use (inter)national law as instrument, and the domestic court as their forum, to seek certain policy changes. One of the points of contention in the Urgenda case was how far a domestic judge could go in evaluating the policy of the government, using such open and vague norms as the duty of care as standard for its evaluation. This question relates directly to the relationship between politics and law, and to the separation of powers between the judicial and political branches – executive and legislative – of the State. Nobody is denying that a decision of a court such as the Urgenda decision can have important implications for policymaking; but does that mean a judge is thereby overstepping the boundaries as established in the trias politica?