Any successful effort to address climate change over the next 25 years will involve a “credible swap” that greatly reduces greenhouse gas emissions from the burning of fossil fuels, provides energy in entirely different ways, and also reduces demand for energy. On the demand side, the next quarter century offers abundant high-leverage opportunities to reduce future emissions via intelligent urban design, building construction (notably in the gigantic apartment complexes), and efficient vehicles and appliances. On the supply side, there are three variants of the swap. In one, the work horse is renewable energy (solar power, wind power, hydropower, and power from biological feedstocks); in a second it is nuclear power; and in a third it is a reshaped fossil fuel economy. The three are by no means mutually exclusive. Each brings disruption and risks, rivaling those of climate change if done inattentively. Well-executed solutions will require threading a needle.