The theory of the stellar interior indicates that for a star of given mass, given initial composition, and given age, all physical characteristics (including the luminosity and the effective temperature) are uniquely determined. Accordingly it should be possible to derive theoretically for a particular star of definite mass and initial composition a relation between luminosity and effective temperature, i.e., a curve in the Hertzsprung-Russell diagram along which the star must move during its evolution. With the help of such evolution curves it must then be possible to identify any observed star as to its mass, composition and age. This identification is possible at present only for certain stellar types, for two reasons. First, the theoreticians have not yet succeeded in deriving all the necessary evolution curves. Second, the evolution curves cross over each other in a complicated pattern in the Hertzsprung-Russell diagram so that observations in the two dimensional Hertzsprung-Russell diagram will not always suffice to identify uniquely the state of an individual star although it appears to suffice for the identification of a whole sequence of stars, like those which are observed in stellar clusters.