The outer disks of spiral galaxies remain a relatively unexplored territory in studies of the evolution of galaxies. This Chapter has highlighted the somewhat unexpected attributes of the ionized gas chemical composition discovered in recent years in the outermost parts of galaxy disks, at least for systems with extended H i envelopes and ongoing star formation. At the same time, it is important to stress that metallicity information in the outer disks, gathered until now almost exclusively from H ii regions, can provide crucial constraints for models of the chemical evolution of galaxies, considering that these are the most recently assembled regions of the disks according to the inside-out scenario. Future work will more firmly establish how common shallow or flat outer gradients with relatively high oxygen abundances are in spiral disks, which is relevant to ascertain the roles played by enriched galactic inflows and mixing mechanisms, such as turbulence, in regulating the chemical evolution of galaxies. Studies of the evolution of galaxies will benefit from probing the relationship between the gas phase chemical abundances and the properties of the faint stellar populations present in these low surface brightness structures. A better understanding of the mechanisms leading to the formation of these extended structures, and of the importance of the galactic environment in this context is highly desidered. From the observational point of view, the combination of deep integral field spectroscopy with spatially resolved radio H i mapping for large samples of spirals will yield a better characterization of the relationship between gas, star formation, environment and metal production in determining the evolutionary status of present-day galaxies out to very large radii.
Acknowledgements The author is grateful to Rob Kennicutt, Emma Ryan-Weber and Rolf Kudritzki for interesting collaborations and stimulating discussions over the years, and to the editors of this volume for the invitation to contribute this Chapter.