The next few decades are likely to witness deep environmental crises, crises we will be able to cope with only through a clear understanding of the complex, delicate system of which we are part. Fortunately, the great advances made in all fields of science since World War II make it possible to reconstruct the entire life history of the world we live in, from the Big Bang to the present, and thus to understand how the system works. This book presents a global picture of our world - how it originated, how it evolved, how it works - and provides the background necessary to assess ways to stabilize it. Although the science is rigorous and quantitative, the book is written in an informal style and is readily accessible to anyone with a knowledge of high-school algebra.