In the twenty-first century people use energy in a wide variety of ways. Energy powers our computers, lights and heats our homes, fuels our transportation, and manufactures the goods we need. Almost all of this energy comes from fossil fuels. As these fuels begin to run out, what will replace them? This book examines our reliance on fossil fuels and their impact on the environment, the pros and cons of nuclear energy, the debate about biofuels from ethanol to algae, harnessing energy from heat from the Earth, the Sun, wind, and water, and energy in the future, from hydrogen fuel cells to solar power from space. This book goes beyond the science to explore the pros and cons of each new development, and examines the ways each energy source has changed our society and the world.