When Parliament passed the ANPS, Friends of the Earth and Plan B Earth teamed up with other campaigners to launch a judicial review into the decision. They argued that the new runway would jeopardise the UK’s ability to achieve its greenhouse gas emission-reduction targets, and that the Paris Agreement had not been considered by the Secretary of State when designating the ANPS. This chapter traces the story of the legal challenge, unpacking the arguments. It shows how the case occurred at a time of change in the UK public’s understanding of climate change, as Extinction Rebellion and Fridays for Future attracted thousands of people demonstrating for radical action. The judgment in the High Court came on the day Parliament declared a climate emergency and shortly before the UK committed to reaching Net Zero by 2050. The chapter documents the dramatic win in the Court of Appeal in 2020, just before COVID-19 devastated the international aviation industry, and explains and analyses why the Supreme Court overturned that judgment. It introduces the work of the UK’s Climate Change Committee on possible pathways for decarbonising aviation, and analyses whether the expansion plans can ever be compatible with reaching Net Zero emissions by 2050.