The photometric (spectral) evolution is but one aspect of a more universal central question in modern astrophysics — the complex problem of formation and evolution of galaxies. The method used to follow the photometric evolution is straightforward, consisting in computing on the basis of adopted prescriptions for star formation the stellar populations present at various moments and summing up the contributions of all the multitude of stars of different ages and evolutionary stages. The usual strategy is to neglect the dynamics and chemical changes of matter, although during the initial collapse of a galaxy and rapid enrichment of its matter both processes should be important contributors shaping the forming stellar populations. Other widely used simplifications are the rejection of interstellar absorption and the restriction of models to the volume element with constant star formation time scale in its borders (so-called “one-zone models”, they stricly apply to the infinitely narrow ellipsoidal shells in real galaxies). An additional postulate that the region considered is closed and isolated, having not been subjected to interactions or mass exchange with the surroundings, is introduced in the simpliest closed “classical” or “canonical” models (Traat 1988), computed e.g. by Tinsley (1972), Searle et al. (1973), Traat and Einasto (1979), and others. Such assumption, however, is not always valid and some objects need to be modelled by burst models or more universal accretion models.