This paper is concerned with the refinement of the quantitative conclusions of the hydrodynamic theory of multiparticle processes which was put forward by Landau 40 years ago. The experimental data obtained by modern accelerators are compared with the predictions and conclusions of the theory. A comparison is made between the model proposed by Landau and the scaling model. The authors point out that in the latter model the initial temperature and energy density values are underestimated. The concepts of the hydrodynamic theory and the quark-gluon theory are compared. The common nature of the two approaches is noted and some discussion is presented regarding ways of developing the two directions. Collisions of heavy nuclei are analyzed.