Several porous silicas have been studied in order to determine the origin of porosity in random porous media. The silica system offers a unique opportunity to study the origin of porous structures because a variety of different structures can be synthesized depending on precursor chemistry and physics. In solution‐grown materials, for example, it is possible to grow particles with randomly rough surfaces, smooth surfaces, as well as polymerlike structures. Porous materials can be made from all these synthetic precursors and the structure of the final product reflects the geometry of the precursors. In addition, porous materials can be made by a phase‐separation/leaching process which gives rise to a completely different type of porosity network. Structures from these various classes have been studied by small‐angle x‐ray and neutron scattering and in some cases it is possible to directly trace the structure of the porous material to the geometry of the precursor macromolecules.