if we can correctly estimate the amount of gas ejected by stars, the fate of this gas goes beyond the possibilities of classical semi-analytical models.

As a third point, note that when we fold many SSPs to calculate a galaxy SED using the classical EPS technique we simply convolve their fluxes with the SFH of the galaxy. Many classical spectrophotometric semi-analytical models of galaxies are built in this way: there is no dust at the level of SSPs and again no dust at the level of the galaxy model [see e.g. @Arimoto87; @Arimoto90; @Bruzual93; @Tantalo96; @Kodama97; @Tantalo98; @Buzzoni02; @Buzzoni05]. To calculate the emission by dust, a higher level of sophistication of the model is required. Indeed one has to develop a model in which the sources of radiation and the emitting/absorbing medium are distributed, to face and solve the problem of the radiative transfer simulating in a realistic way the interactions among the various physical components of a galaxy. Among recent models of this kind are those by @Silva98, @Devriendt99 and @Takagi03.

Improving upon semi-analytical models
-------------------------------------

Two drawbacks of the semi-analytical models concern therefore: (1) the description of galactic wind, which is supposed to occur within a finite time 