  A plasmon of a bounded domain $\Omega\subset\mathbb{R}^n$ is a non-trivial
bounded harmonic function on $\mathbb{R}^n\setminus\partial\Omega$ which is
continuous at $\partial\Omega$ and whose exterior and interior normal
derivatives at $\partial\Omega$ have a constant ratio. We call this ratio a
plasmonic eigenvalue of $\Omega$. Plasmons arise in the description of
electromagnetic waves hitting a metallic particle $\Omega$. We investigate
these eigenvalues and prove that they form a sequence of numbers converging to
one. Also, we prove regularity of plasmons, derive a variational
characterization, and prove a second order perturbation formula. The problem
can be reformulated in terms of Dirichlet-Neumann operators, and as a side
result we derive a formula for the shape derivative of these operators.
