Over its 25 history, the UN Framework Convention on Climate Change (UNFCCC) regime has grown tremendously in size and complexity. Much of this evolution has occurred from the bottom-up, through a series of ad hoc, incremental changes. After years of almost continuous negotiation, the completion of the rulebook for implementing the Paris Agreement presents an occasion to take stock of the UNFCCC’s institutions and functions, and to consider in a more systematic, top-down manner how the regime might evolve as its focus shifts from negotiation to implementation. This paper offers three frames for thinking about climate change governance, reviews the UNFCCC’s current functions, assesses its strengths and weaknesses, and presents three versions of how the regime might evolve.