Fracture of root-filled teeth is not uncommon and appears to be a complex function of both the treatment and the patient's oral regimen. However, there have been few studies aimed at understanding the intrinsic mechanical behavior of dentin and its relevance to the incidence of these fractures. In addition, there has been some controversy over whether the fracture of root-filled teeth is attributed primarily to loss of tooth structure or if there are other contributing factors. A comprehensive understanding of the structure and mechanical behavior of dentin is of substantial importance to the success of endodontic therapy. Specifically, the importance of fatigue on tooth fractures and the resistance of tissues to both the initiation and propagation of cracks have received scant attention. As well, regional variations in these properties and the contribution of changes in microstructure to the fatigue and fracture behavior are not well understood. This article reviews the importance of microstructure on the mechanical behavior of dentin and compares the mechanical properties of tissue from the root and crown. Also, the changes in microstructure with aging are discussed as well as their importance to the incidence of tooth fracture.