This chapter argues that the customary 'no-harm' principle, conceived by the ICJ as a principle according to which States must prevent activities within their jurisdiction from causing extraterritorial environmental harm, 1 applies to the regulation of greenhouse gas emissions (GHG) that are interfering with the global climate system. As a helpful complement to (invaluable) treaty-based obligations, this principle should help put pressure on States and lead them to reduce their GHG emissions.