The discovery of silicatein caused a paradigm shift, since it is the first enzyme which catalyzes the synthesis of a polymeric inorganic molecule from inorganic monomers. Molecular biological, biochemical and cell-biological data showed that the synthesis of siliceous spicules in both demosponges and hexactinellids is enzymatically driven via silicatein. This enzyme exists both intra-spicularly and in the extra-spicular space. It catalyzes the formation of bio-silica constituting the silica lamellae that are formed during the appositional (layer-by-layer) growth of the spicules. The extent of (bio-silica forming) activity of silicatein from the demosponge Suberites domuncula measured in vitro reflects the amount of bio-silica required for the formation of spicules in vivo. It is furthermore summarized that during growth and maturation of the spicules in demosponges a bio-fusion process occurs that results in an intra-spicular sintering of the silica lamellae to form compact silica rods. Finally we report that for the formation of the strong and stiff bio-silica skeleton of sponges a hardening process is required that is (presumably) driven by cell-membrane bound aquaporin channels which allow the removal of water, which is released during the bio-silica polycondensation reaction.