Crystalline bacterial cell surface layer (S-layer) proteins have been optimized during billions of years of biological evolution as constituent elements of one of the simplest self-assembly systems in nature. Isolated S-layer proteins have the intrinsic property to recrystallize into two-dimensional arrays on a broad spectrum of surfaces including silicon, metals and polymers, and to interfaces such as planar lipid films and liposomes. The well defined arrangement of functional groups on S-layer lattices allows the binding of molecules and particles in defined regular arrays. S-layers also represent templates for the formation of inorganic nanocrystal superlattices composed of CdS, Au, Ni, Pt, or Pd.