Questions about the origin and nature of Earth and the life on it have long preoccupied human thought and the scientific endeavor. Deciphering the planet's history and processes could improve the ability to predict catastrophes like earthquakes and volcanic eruptions, to manage Earth's resources, and to anticipate changes in climate and geologic processes. At the request of the U.S. Department of Energy, National Aeronautics and Space Administration, National Science Foundation, and U.S. Geological Survey, the National Research Council assembled a committee to propose and explore grand questions in geological and planetary science. This book captures, in a series of questions, the essential scientific challenges that constitute the frontier of Earth science at the start of the 21st century.