The formalism of the simple model of galactic chemical evolution (GCE) and its main ingredients (stellar properties, initial mass function, star formation rate and gas flows) are presented in this tutorial. It is stressed that GCE is not (yet) an astrophysical theory, but it merely provides a framework in which the large body of data concerning the chemical composition of stars and gas in galaxies may be intrepreted. A few examples illustrating those concepts are provided, through studies of the Solar neighborhood and of the Milky Way's halo.