Single-celled microorganisms such as diatoms and coccolithophores produce inorganic microparticles with genetically controlled hierarchical nanopatterns. Besides serving as paradigms to inspire new routes for materials synthesis, biominerals themselves, particularly diatom biosilica, are increasingly utilized as templates for the synthesis of novel functional materials. Over the past decade, a large variety of methods have been established that allow not only for the attachment or coating of desired materials onto diatom biosilica but also for complete chemical conversion without altering the characteristic micro- and nanoscale morphology. Examples include the synthesis of materials for photonics (surface-enhanced Raman spectroscopy, SERS, extraordinary optical transmission, EOT), ultraresponsive and sensitive gas sensors, gas storage materials, and highly active catalysts. More recently, emerging insight into the cellular mechanisms of biosilica formation has enabled the in vivo functionalization of diatom biosilica through advanced cultivation techniques and genetic engineering. As a naturally renewable material, biominerals hold the promise of serving as an inexpensive and easily available resource for a future nanotechnology-based industry.