Although carbonates contain about 60% of the world's oil reserves, the characterization of carbonate reservoirs remains challenging because of their heterogeneity andcomplex microstructure. Central to the seismic characterization of carbonates is an understanding of the effect of porosity on the elastic moduli and seismic-wave velocities. Figure 1 shows the compressional and shear-wave velocities measured by Rafavich et al. (1984) for limestones and dolomites from the Williston Basin, North Dakota, measured on air-saturated samples. It is seen that although the velocities decrease with increasing porosity, there is considerable scatter of the data. In this article, the possible contribution of variations in pore shape to the elastic moduli of carbonates is discussed, the role of microcracks and fractures is assessed, and differences between the static and dynamic moduli are highlighted.