Energy undergirds civilization and has powered the sweeping economic changes that have transformed the world over the last two and a half centuries. But just as the economy has changed, so has the energy mix that fuels it. The development of the modern world has been a story of evolving new uses for energy and constantly growing energy demand. New forms of energy and new technology to harness that energy have been developed over time, shifting the energy balance and expanding the menu of energy sources.Today there is great focus on the next transition – on the expectation or the possibility of a substantial change in the energy mix. What would be the nature of the changing mix? What would drive it? How fast could it come? Or how long might it take? The answers to these questions will have a profound impact on the global energy system, on producers and consumers alike and on markets everywhere.In this Energy Vision, our aim is to provide a framework for understanding the potential for changes in the energy mix and how an energy transition could unfold. We do so by looking at how energy transitions have unfolded in the past and the factors that brought about these transitions. Rather than attempting to predict the future, we provide a discussion of factors that may drive changes in the energy mix in the coming decades. Although energy efficiency and other demand-side issues are critical to future energy systems, this report focuses on the supply side of the equation – how society will meet its ever-growing energy needs. We addressed energy efficiency in the 2010 Energy Vision, Towards a More Energy Efficient World, entirely devoted to that subject.