  Standard models for the chemical evolution of the Galaxy are reviewed with
particular emphasis on the history of the abundance gradients in the disk. The
effects on the disk structure and metallicity of gas accretion are discussed,
showing that a significant fraction of the current disk mass has been accreted
in the last Gyrs and that the chemical abundances of the infalling gas can be
non primordial but should not exceed 0.3 Z(sun). The distributions with time
and with galactocentric distance of chemical elements are discussed, comparing
the observational data with the corresponding theoretical predictions by
standard models, which reproduce very well the ISM abundances at various
epochs, but not equally well all the features derived from observations of old
stellar objects.
