This research program aims at improving understanding of structure - mechanical function relations of whole human teeth and adult human osteonal bone. The holistic approach to understanding tooth function is inspired by the observation made by the applicant that a zone some 200 microns thick beneath the dentino-enamel junction (DEJ) absorbs most of the applied stress. In essence it functions as a cushion separating the stiff enamel cap from the bulk crown dentin. Proposed is to develop and apply two complimentary new methods for mapping strain in tooth crowns, and relating this to structure. Strain will be mapped as a function of humidity variations using an environmental scanning electron microscope and holographic interferometry to map strain variations when whole teeth are subjected to compressive forces similar to those in mastication. Also to be investigated are the effects of cavities and restorations on strain distribution. Variations in sub-layer thicknesses of a lamellar unit within an osteonal bone will be investigated to determine if this is the variable that reflects prevailing stress fields at the time of formation. osteonal bone differs in mechanical behavior from circumferential lamellar bone mainly in the manner in which osteonal bone absorbs damage and fractures. The mechanical properties of small portions from different parts of osteonal bone of humans of different ages will be studied by drilling out cylinders of 100 to 200 microns in diameter and measuring their elastic and other properties. The aim is to understand the extent of variation within a bone, and the structural basis for the measured variations. The results will improve understanding of the aging process of human osteonal bone, and will allow better evaluation of possible effects that treatments for osteoporosis may have on mechanical function of bone.