At the end of 2015, in Paris, parties to the United Nations Framework Convention on Climate Change (UNFCCC) will lay the foundation for climate action in a post-2020 world by adopting a new, universal and legally binding agreement. But the event that will be a key determinant of success in Paris takes place one year before: the twentieth session of the Conference of the Parties (COP 20) in Lima, Peru. There, parties will need to make significant progress on four issues to keep negotiations on track to meet agreed deadlines: to determine how each party should contribute to the new agreement; to advance on all core elements of the agreement; to agree on its legal form; and to double efforts to reduce greenhouse gas emissions in 2015–2020.