The review papers in this series were originally published in English in 1980s and were written by the very best experts in the Soviet Union. and are now regarded as classic papers in particular areas of physics. The beginning of the 1980s was a time of active development in physics and discoveries included anisotropic superfluidity in liquid helium, high temperature superconductivity, theory of phase transitions, theory of superstrings. Classic Reviews in Physics  publishes and highlights the best examples of classic papers which are intended as a useful source of reference for postgraduate students and researchers.