  We study resonance for the Helmholz equation with a finite frequency in a
plasmonic material of negative dielectric constant in two and three dimensions.
We show that the quasi-static approximation is valid for diametrically small
inclusions. In fact, we quantitatively prove that if the diameter of a
inclusion is small compared to the loss parameter, then resonance occurs
exactly at eigenvalues of the Neumann-Poincar\'e operator associated with the
inclusion.
