This unique book presents an overview of isostasy, a phenomenon of fundamental importance to the earth sciences. Isostasy is a simple concept, yet it has long perplexed students. To overcome this problem, the author uses a simplified mathematical treatment, numerous geological examples and an extensive bibliography. The book begins by tracing the ideas behind local and regional models of isostasy, arguing that only flexure is in accord with geological observations. It then proceeds to describe the theoretical background, the observational evidence and the constraints that flexure has provided on physical properties of the lithosphere. The book concludes with a discussion of flexure's role in understanding the evolution of the surface features of the Earth and its neighboring planets.