Gold-core/silver-shell nanoparticles featuring core diameters of 13 or 25 nm were prepared in aqueous solution. Silver-coated gold nanoparticles have two distinct plasmon absorption bands whose relative intensities depend on the shell thickness-features that are captured well by classical Mie theory. The core/shell spectra are not simply linear combinations of the pure-component spectra. Theory shows that shell dielectric effects upon the core can cause broadening, shifting, and, in some cases, damping of dominant extinction spectral features. Preliminary studies show that core-shell particles are reasonably efficient hyper-Rayleigh scatterers under conditions of two-photon resonance or near resonance with a high-energy plasmon band. (C) 2002 Published by Elsevier Science B.V.