Spectra and essential spectra of selfadjoint and essentially selfadjoint operators.- Schrödinger operators.- Perturbations small at infinity.- Examples.- Operators acting only on part of the variables.- N-particle Hamiltonians.- Symmetries of the Hamiltonian.- The spectrum of the Hamiltonian of a free system.- A lower bound of the essential spectrum.- The essential spectrum of the Hamiltonian of an N-particle system with external forces.- The essential spectrum of the internal Hamiltonian of a free system.- Proof of theorem 11.16.