Global properties of stellar populations and the spectral evolution of galaxies.

Several relevant properties of evolving stellar populations are discussed from a theoretical viewpoint. The authors introduce the concept of phase transition in the stellar content of a population, an event which is produced by the first appearence of stars with either C-O or He degenerate cores. The spectral energy distribution of a population is then predicted to suffer a (major) change at each of these phase transitions. Some uncertainties in both current stellar and population evolutionary models, and in population synthesis techniques are then briefly discussed, and the need is emphasized for appropriate checks and calibrations using template stellar populations.