A key outcome of the Seventeenth Conference of the Parties (COP-17) of the United Nations Framework Convention on Climate Change (UNFCCC), held in Durban, South Africa, late in 2011 — the Durban Platform for Enhanced Action — represents an important milestone in the history of climate negotiations. This is because it departs from the long-standing and problematic dichotomous division of the world's countries into those with serious emissions-reduction responsibilities and the others — with no such responsibilities whatsoever. That distinction, now apparently abandoned, has prevented meaningful progress for decades. The Durban Platform — by replacing the Berlin Mandate's (1995) division of the world into a set of countries with ambitious responsibilities and another set of countries with no responsibilities — has opened an important window. National delegations from around the world now have a challenging task before them: to identify a new international climate policy architecture that is consistent with the process, pathway, and principles laid out in the Durban Platform, while still being consistent with the UNFCCC. The challenge is to find a way to include all key countries in a structure that brings about meaningful emission reduction on an appropriate timetable at acceptable cost, while recognizing the different circumstances of countries in a way that is more subtle, more sophisticated, and — most important — more effective than the dichotomous distinction of years past.