Our bones are full of cracks, which form and grow as a result of daily loading activities. Bone is the major structural material in our bodies. Although weaker than many engineering materials, it has one trick that keeps it ahead-it can repair itself. Small cracks, which grow under cyclic stresses by the mechanism of fatigue, can be detected and removed before they become long enough to be dangerous. This article reviews the work that has been done to understand how cracks form and grow in bone, and how they can be detected and repaired in a timely manner. This is truly an interdisciplinary research field, requiring the close cooperation of materials scientists, biologists and engineers.