  We present a survey of some recent results concerning the location and the
Weyl formula for the complex eigenvalues of two non self-adjoint operators. We
study the eigenvalues of the generator $G$ of the contraction semigroup
$e^{tG}, \: t \geq 0,$ related to the wave equation in an unbounded domain
$\Omega$ with dissipative boundary conditions on $\partial \Omega$. Also one
examines the interior transmission eigenvalues (ITE) in a bounded domain $K$
obtaining a Weyl formula with remainder for the counting function $N(r)$ of
complex (ITE). The analysis is based on a semi-classical approach.
