Our understanding of how the skeleton is remodeled and repaired has been enormously expanded in the past decade. Using advanced molecular technologies including transgenesis, gene knockout, and gene array, we are gaining a better understanding of both the precise pathways through which osteoblasts lay down new bone and how osteoclasts remove old bone. These studies have not only helped us understand the basic biological properties of the skeleton and how it responds to hormonal, cytokine, and mechanical stimulation, but also, in some instances, clarified the source of pathophysiology. The eventual goal of such discoveries is to lay down a firm scientific foundation for the identification of novel cellular and molecular targets for future drug development. This volume presents the significant headway that scientists have made in this undertaking and the book is organized around four broad themes: skeletal development and repair, molecular endocrinology of bone, bone cell biology, and conservation of skeletal integrity.