A new generation of spectroscopic dyes is gradually becoming available to biological researchers, from an unexpected source: materials chemists who study the synthesis and properties of nano-sized inorganic objects. Research into tailoring the optical properties, surface chemistry and biocompatibility of metallic and semiconductor nanoparticles, exemplified in part by a recent report by Mirkin, Schatz and coworkers, is fulfilling the promise of these nanostructures as customizable substitutes for organic molecular probes.