Biomineralisation is widespread in the biological world and occurs in bacteria, single-celled protists, plants, invertebrates and vertebrates. Minerals formed in the biological environment often show unusual physical properties (e.g. strength, degree of hydration) and often have structures that exhibit order on many length scales. Biosilica, found in single cell organisms through to higher plants and primitive animals (sponges), is formed from an environment that is undersaturated with respect to silicon and under conditions of around neutral pH and low temperature, ca. 4-40 degrees C. Formation of the mineral may occur intra- or extra-cellularly, and specific biochemical locations for mineral deposition that include lipids, proteins and carbohydrates are known. In most cases, the formation of the mineral phase is linked to cellular processes, understanding of which could lead to the design of new materials for biomedical, optical and other applications. This Chapter briefly describes the occurrence of silica in biology including known roles for the mineral phase, the chemistry of the material, the associated biomolecules and some recent applications of this knowledge in materials chemistry.The terminology which is used in this and other contributions within this volume is as follows: Si: the chemical symbol for the element and the generic term used when the nature of the specific silicon compound is not known. Si(OH) ( 4 ): orthosilicic acid, the fundamental building block used in the formation of silicas. SiO ( 2 ) x nH ( 2 ) O or SiO ( 2-x ) (OH) ( 2x ) x 2H ( 2 ) O: amorphous, hydrated, polymerised material. Oligomerisation: the formation of dimers and small oligomers from orthosilicic acid by removal of water. For example, 2Si(OH)(4) <--> (HO)(3)Si-O-Si(OH)(3) + H(2)O Polymerisation: the mutual condensation of silicic acid to give molecularly coherent units of increasing size. Organosilicon compound: must contain silicon covalently bonded to carbon within a distinct chemical species Silane: a compound having silicon atom(s) and organic chemical groups often connected through an oxygen linkage; e.g. tetrethoxy or tetramethoxysilane Silanol: hydroxyl group bonded to silicon atom Silicate: a chemically specific ion having negative charge (e.g. [Formula: see text]), term also used to describe salts (e.g. sodium silicate Na(2)SiO(3)) Opal: the term used to describe the gem-stone and often used to describe the type of amorphous silica produced by biological organisms. The two are similar in structure at the molecular level (disordered or amorphous), but at higher levels of structural organisation are distinct from one another.