“…That spectral instability is not only a nuisance, but can be at the origin of nice and previously unexpected results, will hopefully be clear from the second and main part of these lecture notes, where we shall describe some results about Weyl asymptotics of the distribution of eigenvalues of elliptic operators with small random perturbations. These results have been obtained in recent works by M. Hager [36,37,38], Hager and the author [39], W. Bordeaux Montrieux [10], the author [97,98] and Bordeaux Montriex and the author [11].…”