Most people know that Einstein's equation is important but they don't usually know why. This book takes an approach to the equation that concentrates not on the biography of Einstein but on the biography of the equation itself. David Bodanis looks at the ancestors of the equation, the three elements - e, m and c - before they end up together in Einstein's equation in Berne, 1905. From there he follows the course of the equation through the 20th century, focusing on the people who developed Einstein's work and its consequences. Without the equation for instance there would have been no atomic bomb, no lasers, no Internet and no science of black holes.