The optical properties of plasmon resonant metallic nanoparticles are of great interest because of their ability to both control optical fields on the nanometer scale and function as sensitive indicators of their local environment. We investigate the relationship between the dielectric function of a metal and the optical properties of the constituent nanoparticle. Using a Drude shell-silica core nanoshell geometry, we examine how systematic changes in the parameters of the Drude dielectric function affect the near and far field properties of the nanoparticle. The nanoshell geometry allows separation of intrinsic properties and extrinsic phase retardation, or finite size, effects. (C) 2004 Elsevier B.V. All rights reserved.