The Deep Decarbonization Pathways Project (DDPP) is a collaborative global research initiative to understand how individual countries can transition to a low-carbon economy consistent with the internationally agreed goal of limiting anthropogenic warming to less than 2 degrees Celsius (°C). Staying within this limit requires global net emissions of greenhouse gases (GHG) to approach zero in the second half of this century. This will entail, more than any other factor, a profound transformation of energy systems, through steep declines in carbon intensity across all sectors, a transition we call “deep decarbonization”.