Dentin consists of several identifiable structures: tubules with cell processes and fluid; highly mineralized peritubular dentin; and intertubular dentin consisting mainly of collaen and deposited apatite. The structural organization and microstructural variations reflect formative influences, such as tooth size and shape, and alterations caused by age, insult, and disease. However, details of structure-chemistry-property relationships for this anisotropic biologic composite are limited. Materials scientists are focusing on an array of new spectroscopic, analytical, and imaging techniques that are yiel ding improved understanding of structural variations and their dependence on specimen preparation, tooth type, location, storage conditions, and chemical and physical modifications. Such studies should lead to the major refinements in structure-properties models for dentin that are required for continued advances in dental composite resins and adhesives.