A review is given of methods involving two fundamentally different approaches to the problem of isotope separation. The first approach is based on selective action on definite isotopic modifications of atoms and molecules. These are primarily laser methods of isotope separation: separation by means of light pressure in two-step schemes of excitation of atoms and molecules, and so forth. Special attention in the review is devoted to the second approach to isotope separation, which is based on selective action on the conditions of occurrence of processes in which a difference in the properties of isotopic molecules appears. The possibility is demonstrated of efficient separation of isotopes in processes with participation of nonequilibrium (vibrationally) excited molecules. In this case the isotopic selectivity of the processes is associated with the specific properties of the exchange of vibrational energy between isotopic molecules.