The attachment between cementum and dentin has been given several definitions and nomenclature, including: interzonal layer, intermediate cementum, collagen hiatus, Hopewell-Smith's hyaline layer, and more commonly, cementum-dentin junction (CDJ). Understanding the attachment of two structurally dissimilar hard tissues such as cementum and dentin defined by a junction may provide information necessary to engineer functionally graded materials that can be used for efficient tooth restorations in clinical dentistry and other bioengineering applications. Hence, in this study, as a first step toward understanding the CDJ using a biomechanical approach, it was hypothesized that the CDJ between cementum and dentin is a wide zone with mechanical properties significantly lower than the neighboring tissues. The structure of the CDJ was studied using an atomic force microscope (AFM), and site-specific mechanical response of the three regions; cementum, CDJ, and dentin were determined using an AFM-nanoindenter under dry and wet conditions. The AFM results of the CDJ demonstrated a valley under dry conditions and a peak under wet conditions. The magnitude of the depth of the valley was approximately the same as the height of the peak of the CDJ, ranging from 10 to 40 microm. The nanomechanical properties under dry conditions indicated no significant difference (p > 0.05) in elastic modulus and hardness of the CDJ (Er = 17.5 +/- 2.7 GPa, H = 0.6 +/- 0.1 GPa) and cementum (Er = 18.7 +/- 2.5 GPa, H = 0.6 +/- 0.1 GPa). The mechanical properties of the CDJ were significantly lower (p < 0.05) than dentin (Er = 19.9 +/- 2.9 GPa, H = 0.6 +/- 0.1 GPa) under dry conditions. However, under more relevant hydrated conditions, the mechanical properties of CDJ (Er 3.0 +/- 0.7 GPa, H = 0.1 +/- 0.0 GPa) were significantly lower (p < 0.05) than those of cementum (Er 6.8 +/- 1.9 GPa, H = 0.2 +/- 0.1 GPa) and dentin (Er 9.4 +/- 2.3 GPa, H = 0.3 +/- 0.1 GPa). Based on the results from this study, it can be concluded that the CDJ can be regarded as a wide zone containing large quantities of proteins including collagen that contribute to hydration and significantly reduce mechanical properties, compared with the adjacent hard tissues, cementum, and dentin. The lower mechanical properties of the CDJ may make it possible for it to redistribute occlusal loads to the alveolar bone. Copyright 2003 Wiley Periodicals, Inc. J Biomed Mater Res 68A: 343-351, 2004