  Spiral star-forming galaxies are complex astrophysical objects whose baryonic
component is dominated by the disk, where most of the star formation resides.
The metallicity in the disk is not uniform, and it usually decreases with the
distance to the galaxy center, in the so-called radial metallicity gradient.
Radial metallicity gradients have been successfully used to set important
constraints on galaxy formation and their chemical evolution. This paper
focuses on the implications of radial metallicity gradients measured with
different probes for a variety of galaxies, and on the foreseen advances in
this field in the astronomical landscape of the 2020s.
