The collection shows the results of theoretical studies on phase transitions in ferroactive and other systems carried out at the Latvian State University, the Riga Polytechnic Institute and the A.F. Ioff. Institute of Physics and Technology. Part of the collection material reflects international relations and the results of joint research of employees of the Department of Theoretical Physics of Leningrad State University named after P.Stutki with scientists of Czechoslovakia On its subject, the collection is devoted to the following main groups of questions: 1) general questions of the theory of phase transitions in condensed systems, 2) phase transitions in films and size effects, 3) fluctuating and phase states of electrons in ferroactive materials, 4) solitons in ordered systems in the vicinity of the phase transition, 5) complex questions of the theory of phase transitions.