Sponges (phylum Porifera), which arose at the dawn of the evolution of multicellular organisms, are a unique storehouse of the biosilica-based bioarchitectures represented by their skeletal formations. The diversity of size (from micro- to meter-long dimensions), shape (up to 100 morphotypes), porosity (from nano- to macro-scale), and hierarchical ornamentation of their interconnected siliceous networks remain to be in a focus of modern materials science and biomimetics. Principles of templating and scaffolding activities of polysaccharide- and protein-based biosynthesis of highly structured biosilica in sponges will be critically analysed in this chapter. Special attention is paid to fundamental role of actin in patterning of sophisticated, branched geometries of poriferan biosilica. Also discussed are prospects for using fundamental knowledge about the hierarchical architecture of biosilicates in sponges to create new functional and sustainable composites.