Porous media consists of materials constituted of a matrix - usually solid - which contains a substantial fraction of voids or pores. The void fraction, i.e. the porosity, dictates the properties of the bulk material. Often the pores are filled with some fluid, like a liquid or a gas. Typical porous materials include different kinds of foams. Many geological materials, like rocks or soil, as well as man-made granular materials like cement and also ceramics, can be considered as porous.