  The basic principles underlying galactic chemical evolution and the most
important results of chemical evolution models are discussed. In particular,
the chemical evolution of the Milky Way galaxy, for which we possess the
majority of observational constraints, is described. Then, it is shown how
different star formation histories influence the chemical evolution of galaxies
of different morphological type. Finally, the role of abundances and abundance
ratios as cosmic clocks is emphasized and a comparison between model
predictions and abundance patterns in high redshift objects is used to infer
the nature and the age of these systems.
