This section will define mercury porosimetry and explain its relevance in the study of the porous structure of cementitious materials. It will describe how mercury is intruded into the pores of hardened concrete under controlled pressure to determine pore size distribution, total porosity, and pore connectivity, which are critical for understanding the material’s permeability and durability. The mechanisms by which mercury interacts with the concrete matrix and the implications of these interactions for interpreting porosimetry data will be detailed.