In this chapter, we describe the properties of irregular galaxies and the proposed chemical evolution models. Irregular galaxies are generally small and rather simple objects with low metallicity and large gas content, suggesting that they are either young objects or have undergone discontinuous star formation activity (bursts) or a continuous but not efficient star formation. They are very interesting objects for studying galaxy evolution: in fact, in “bottom-up” cosmological scenarios, dwarf galaxies have been suggested to be the first self-gravitating systems to form. Moreover, dwarf irregular galaxies could also be important contributors to the population of systems giving rise to QSO-absorption lines at high redshift, in particular the damped-Lyman-α systems.In this chapter, we also show the mass–metallicity relation found for star forming galaxies at low and high redshift and discuss some possible interpretations.