In order to understand and evaluate any proposal, such as the Obama EPA’s Clean Power Plan, to shift American electricity generation to rely more heavily on renewable power, one must first understand the basics about how the US electricity system works. As we shall see in this chapter, moving to a higher share of renewable power while ensuring the reliability of electricity supply is far from trivial. The cost of a system with a high renewable power share is far greater than the cost of simply building and operating wind and solar farms. It includes not only the costs of building new transmission lines, and upgrading transmission stations and substations, but also the cost of electricity from rapidly dispatchable power sources such as natural gas turbines that can provide power when solar and wind power are unavailable. Together, these costs cause high electricity prices in systems with high renewable power penetration.