ABSTRACT.    Since their discovery, about five thousands years ago, silica glasses have been made from molten sand and processed at high temperature. However, some micro-organisms such as diatoms are able to produce silica exoskeletons to protect themselves against predators. They exhibit highly sophisticated shapes and offer a real challenge to materials scientists. Nanostructured diatom frustules are being used for making 3D genetically engineered micro/nano devices (3D-GEMS). Gas sensors based on the electrical or optical properties of silica have been made. Moreover, diatoms have been shown to behave as living photonic crystals. The shape of the frustules can be preserved upon the chemical transformation of silica into other oxides (TiO2, ZrO2,...) and even silicon. Biogenic silica is synthesized at room temperature, from the very small amount of silica dissolved in the sea water. Therefore, during the past decades, chemists have developed new routes based on soft solution chemistry in order to produce nanostructured materials. The silica network is synthesized via the polycondensation of solute molecular precursors such as silicic acid Si(OH)₄ or silicon alkoxides Si(OR)₄. One of the main advantage of "chimie douce" is to allow the synthesis of hybrid organic-inorganic materials in which both organic and inorganic phases are mixed at the molecular level. These hybrids cover the whole range of materials from plastic polymers to brittle glasses. They are highly transparent and find applications for the realization of optical devices in which organic dyes are trapped within a silica glass. Recently, biomolecules such as proteins, enzymes or antibodies have been immobilized within sol-gel silica. They have been shown to retain their bioactivity and can be used for making biosensors or bioreactors. Antibodies and even whole cells have also been trapped within sol-gel matrices. One of the main challenge of sol-gel bio-encapsulation is the viability of micro-organisms inside the silica matrix. Recent studies show that bacteria may remain viable for several weeks within silica gels. They keep their metabolic activity and could be used for the production of drugs. Cell transplantation assays are also being performed for medical applications.