This paper contains an exposition of the classical hydrodynamical interpretation of multiple hadron production processes. Special attention is given to the clarification of the fundamental aspects of the hydrodynamical theory, the analysis of nucleon-nucleus collisions, and e$^+$e$^-$ annihilation into hadrons. The conclusions of the theory are compared with experimental results. Some heuristic aspects of the theory are outlined. Use is made of the literature up to the beginning of 1975.