The role of judges in implementing climate policies has become a crucial component of the existing governance framework regulating climate change action. Litigation focusing on more ambitious climate action is trending globally. Individuals, local authorities and NGOs are bringing lawsuits against national governments, holding them accountable to their legal obligations and engendering policy change. Due to the constitutional doctrine of the separation of powers, the justiciability of climate policy is questioned. Disagreements exist between advocates of an activist judicial role and those in favour of legislative and executive discretion. The main question is to what extent the judiciary can oblige other government branches to take urgent preventative action, particularly to implement or adjust climate policies. Their role in implementing climate policies is analysed from a comparative perspective, considering theoretical debates on the doctrine of the separation of powers in different legal systems and relevant case-law. The chapter connects international and domestic issues and highlights recommendations to foster effective implementation of more ambitious climate policies.