The progress in the investigation of the spectrum-luminosity diagram. The spectrum-luminosity diagram, or the Hertzsprung-Russell diagram, is of extreme importance in stellar astronomy, astrophysics and stellar cosmogony. A number of stellar characteristics can be read from the diagram, as well as the theoretical paths of stellar evolution. A very significant problem is the improvement of the diagram, i.e. the accumulation of more precise data concerning the subdivision into sequences, the contours of the latter (mean lines, dispersions, gaps) and other characteristics (masses, axial rotations, chemical composition, space-kinematical properties).