  Radial metallicity gradients are observed in the disks of the Milky Way and
in several other spiral galaxies. In the case of the Milky Way, many objects
can be used to determine the gradients, such as HII regions, B stars, Cepheids,
open clusters and planetary nebulae. Several elements can be studied, such as
oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other
elements in cepheids, open clusters and stars. As a consequence, the number of
observational characteristics inferred from the study of abundance gradients is
very large, so that in the past few years they have become one of the main
observational constraints of chemical evolution models. In this paper, we
present some recent observational evidences of abundance gradients based on
several classes of objects. We will focus on (i) the magnitude of the
gradients, (ii) the space variations, and (iii) the evidences of a time
variation of the abundance gradients. Some comments on recent theoretical
models are also given, in an effort to highlight their predictions concerning
abundance gradients and their variations.
