We have applied NMR Imaging and X_Ray Computerized Tomography to the study of the structural properties of rocks. Samples from different porous rocks; sandstones, granites, limestones have been successively examined by both techniques. NMR images have been obtained on water saturated samples. The spatial distribution of liquid indicates the effective porosity. By constrast, X_Ray images display the mineral content of rocks. Standard tomographs do not have the required resolution to see pores smaller than 100 μm. We used water as a constrast agent to localize porosities by “differential” CT. Comparative results are shown.