In the last years, we have obtained and analysed a collection of deep optical spectra of a large number of HII regions in the Milky Way, MagellanicClouds and other nearby galaxies, taken with VLT and GTC telescopes. These data have permitted to determine the spatial abundance distribution of several elements with unprecedented precision. An important result of this study is the estimation of Emetallicity variations throughout the discs of the galaxies in order to know the degree of homogeneity and mixing in the ionized gas phase of the interstellar medium (ISM). From our data, we find that the amplitude of such variations are of the order of the errors of abundance determinations of individual objects, indicating the absence of large inhomogeneitiesalong the whole disc in the case of the Magellanic Clouds and at a given galactocentricdistance, in the case of the Milky way and other spiral galaxies. Our results for the Milky Way contrast with recent metallicity estimations in neutral clouds of the local Galactic thin disk that found metallicity variations of the order of a factor ten. We compare the results obtained for neutral clouds with those for HII regions and other tracers of the chemical composition of the ISM as B-type stars, classical Cepheids and young clusters.