The aim of this collection is to make available in English recent results obtained by Soviet scientists in low-temperature physics. The selection of review papers for this collection was guided by two factors. Firstly, all of them deal with closely related subjects: most of the papers are devoted to superconductivity. The superfluidity of helium II discussed in one paper and superdiamagnetism considered in another are both associated with superconductivity to a certain extent. Secondly, all but one of the authors are on the staff of the I.E. Tamm Department of Theoretical Physics at the Lebedev Physical Institute of the USSR Academy of Sciences. This department, which celebrated its fiftieth anniversary in 1984, is one of the largest departments of theoretical physics not only in the USSR but in the whole world. Many fields of scientific activity are being developed here, the solid state physics and, in general, the condensed state of matter occupying the leading position among them. It is this branch of physics to which the problems discussed in this collection obviously pertain.