Diatoms account for 25% of global primary productivity and are the base of many marine and freshwater food webs. They are encased in a porous silica matrix called a frustule, which within a species is reproduced exactly down to nanometres, but among species can be highly variable in terms of the morphology. The function of these nanometre structures is unknown. Diatoms are the most diverse unicellular microalgae with more than 100 000 known species. This diversity is reflected in and possibly caused by the porous outer silica coating known as the frustule. The frustule maintains cell shape, functions as a protective barrier, is a porous nutrient absorber, creates chemical nanoenvironments, provides structural defence against attackers and optically modifies impinging light fields. This wide variety of functions may explain the wide variety of nanostructures that permit the identification of so many tens of thousands of species with distinctive morphologies. However, to date there has been no review on the variety of structures. Here we present a review of the function of the diatom frustule and use the literature to highlight possible uses from understanding these functions. These structures offer the opportunity to understand how flow and chemical flux are modified at the nanoscale.