The importance of knowing the pore size distribution of a material, and the utility of mercury intrusion, are briefly reviewed. Mercury intrusion results can be used to obtain the specific surface of a material. Under the right circumstances, this method has many advantages over vapor sorption, and a mercury intrusion test may be desirable for this reason alone. The procedure and limitations are described. Modern instruments allow for the measurement of mercury extrusion as well as intrusion. This opens the possibility of measuring two pore size distributions. One is reversibly intruded, and the other is not. The use of these sub-distributions in seeking correlations with material properties and behavior is discussed. The applied-pressure/diameter-intruded relationship of the Washburn equation depends upon the surface tension and contact angle. Inter-metallic solutions with mercury as the solvent offer the possibility of reducing the surface tension and/or the contact angle. This allows the use of lower pressures to intrude the same diameter pore, or the intrusion of smaller pores at the same pressure. The practical application of using such solutions is described.