A quantitative introduction to the energy system and its environmental impacts. Humanity faces an immense challenge: providing abundant energy to everyone without wrecking the planet. If we want a high-energy future while protecting the natural world for our children, we must consider the environmental consequences of energy production and use. But money matters too: energy solutions that ignore economic costs are not realistic—particularly in a world where billions of people currently can’t afford access to basic energy services. How can we proceed?