  We present models of photometric evolution of galaxies in which the effects
of a dusty interstellar medium have been included with particular care. A
chemical evolution code follows the star formation rate, the gas fraction and
the metallicity, basic ingredients for the stellar population synthesis. The
latter is performed with a grid of integrated spectra of simple stellar
populations (SSP) of different ages and metallicities, in which the effects of
dusty envelopes around asymptotic giant branch (AGB) stars are included. The
residual fraction of gas in the galaxy is divided into two phases: the star
forming molecular clouds and the cirrus. The relative amount is a model
parameter. The molecular gas is sub--divided into clouds of given mass and
radius: it is supposed that each SSP is born within the cloud and progressively
escapes it. The emitted spectrum of the star forming molecular clouds is
computed with a radiative transfer code. The cirrus emission is derived by
describing the galaxy as an axially symmetric system, in which the local dust
emissivity is consistently calculated as a function of the local field
intensity due to the stellar component. Effects of very small grains, subject
to temperature fluctuations, as well as PAHs are included. The model is
compared and calibrated with available data of normal and starburst galaxies in
the local universe, in particular new broad--band and spectroscopic ISO
observations. It will be a powerful tool to investigate the star formation, the
IMF, supernovae rate in nearby starbursts and normal galaxies, as well as to
predict the evolution of luminosity functions of different types of galaxies at
wavelengths covering four decades.
