Professor John Odell examines the Paris Agreement from a negotiation theory perspective. Focusing on the question of why the agreement was finally reached in 2015, Odell identifies four major explanations. First, as the scientific consensus in support of human-driven climate change strengthened, parties learned that alternatives to the carbon economy were both necessary and feasible. Second, non- and sub-state actors mobilized in greater numbers and with greater force than before, offsetting resistance at the domestic level. Third, between 2011 and 2013, parties coalesced around a less demanding negotiation objective, making it easier for states to join in support. Fourth, adroit conference leadership, in particular by the French Presidency, reduced mistrust and contributed to avoiding deadlocks that had scuttled earlier negotiations. Odell concludes that the four primary reasons are probably best understood as necessary conditions – in other words, in the absence of any of these factors, the substantive shift in policy that the Paris Agreement represents would have been delayed or not happened at all.