To observe and quantify the production of microfracturing from initial yield to failure, we deformedCarrara marble samples in uniaxial compression at 20, 105, and 180°C and continuously observed a region ofabout 1 mm2on an exposed face with a long-working distance microscope. Using image processing andmicroscale strain-mapping techniques, we measured local strains over a length scale of tens of micrometers.By treating the images with various filters, we identified linear damage features, as well as the magnitude ofloc alize d strain and the mode of deformation, i.e., shear versus normal deformation. In general, sheardeformation is more prevalent after initial yielding, while tensile deformation dominates closer to peak stress.Independent measurements of both stress and microcrack density at different stages of each experimentprovide a unique opportunity to explicitly compare the data with damage models. The model of Ashby andSammis (1990) significantly underestimated the damage that the rock could sustain before peak stress, perhapsowing to the influence of weak grain boundaries on the damage production. In these samples, microcrackstended to form near boundaries before yield stress. During strain hardening, the damage parameters increasedrapidly as lon ger microcracks grew along the boundaries and finally transected grains as loading neared peakstress. The microcrack density can be empirically related to the reduction of Young’s modulus; stiffness ratiosdecay exponentially with increasing microcrack density for T ≤ 105°C.