Changing Energy outlines how humanity came to its current energy economy through three previous energy transitions and now stands poised for a necessary fourth one. Despite the immense benefits conferred by a global energy economy based primarily on coal, oil, gas, and uranium, societies must now rebuild their energy economies to rely as much as possible on renewable energy used efficiently. This imperative to change comes from the risks of climate change plus the dangers of geopolitical tensions, health and environmental effects, and the long-term prospects for ever depleting sources of today’s energy sources. Changing Energy argues that sustainability of the benefits from energy services will come from investments made in the technologies of the fourth transition. Perkins envisions a viable post–fossil fuel energy economy and outlines the barriers that must be resolved to reach it.