Diatoms are eukaryotic, unicellular algae that are ubiquitously present in almost any water habitat on earth. Diatoms dominate phytoplankton populations and algal blooms in the oceans. They are responsible for about 25 % of the world's net primary production. Apart from this ecological significance, diatoms are mainly known for the intricate geometries and spectacular patterns of their silica-based cell walls. These patterns are species specific. They are precisely reproduced in each generation documenting a genetic control of this biomineralization process. Biogenesis of the diatom cell wall is considered to be a paradigm for the controlled production of nanostructured silica. Biochemical studies demonstrated that diatom biosilica is a composite material containing zwitterionic proteins (silaffins) and long-chain polyamines in addition to silica. Functional studies indicate a crucial role of these organic components in guiding silica precipitation as well as in the formation of species-specific nanopatterns. These activities can be explained by molecular self-assembly and phase-separation processes. Moreover, diatom cell walls also exhibit very exciting properties from the physical point of view: they are extremely stable and they may act as photonic crystals.