For centuries diatoms have been admired by microscopists for the unusual shapes and intricate, lace-like patterns of their silica-based cell walls. Only comparatively recently, biochemical studies have identified frustule-specific organic biomolecules, of which many are novel and often display remarkably complex chemical structures. This chapter summarizes the current knowledge about the structures and properties of the frustule-associated biomolecules, with a focus on the proteins, and discusses their roles with regards to silica biogenesis and frustule function.