In the last decade, a number of significant results have been obtained in the perturbation theory for selfadjoint and nonselfadjoint operators. In particular, several important results have been gained in the theory of nonselfadjoint perturbations of selfadjoint operators. v1e should mention, for example, the various theorems concerning the completeness of root vectors which have been proven by Soviet mathematicians, starting with the well-known work of M. V. Keldysh.In the present series of lectures, we shall discuss some investigations into the theory of selfadjoint perturbations of selfadjoint operators with arbitrary spectra. The motivating force behind these investigations comes entirely from physicists, who advanced many new, and sometimes most paradoxical ideas. At this time, mathematicians have joined in the effort of elaborating these ideas. As a result, a rather "formidable" theory has been constructed, which cannot be covered in a few lectures. This is why the aim of the present series of lectures is to explain (not always with complete proofs) the fundamental propositions of this theory, and to formulate a number of open problems.In all probability, these lecture notes would not have -------