The yearly world production of biogenic silica of over 20 billion tons play a crucial role in the global carbon cycle and can even affect atmospheric CO2 concentration. About three fourth of this amount is produced in seawater by diatoms, which are small photosynthetic algae, and to a much lesser degree by the radiolarians of the zooplankton or by sponges. The remaining one fourth is produced on land by plants from most groups as concretions named phytoliths. This production has a definite economic importance in terms of diatomite products and of the silica that can be recovered from the ashes of plants such as rice. This biological production involves room-temperature polymerization of silicic acid molecules Si(OH)4 forming at very low concentration in water through silicate dissolution. It has opened wholly new areas in materials science as nanostructured biogenic glasses exhibit very interesting mechanical and optical properties that are the source of a bio-inspiration in sol–gel processes. The frustules of diatoms can be used as precursors for making electroluminescent devices and solar cells and they have a 3-d hierarchical distribution of their pores that may ensure photonic-crystal properties. The soft conditions associated with this process lead to the formation of hybrid nanocomposite materials in which organic and inorganic components are mixed at the molecular scale.