The analysis of the chemical abundance composition of galaxies provides essential and unique constraints on their evolutionary status and their star formation properties. Gathering spatially resolved information about the distribution of metals is a well-tested approach to probe not only the metal production in stars across time, but also those effects, such as galactic wind outflows, gravitational interactions, secular processes and gas inflows, that can profoundly affect the evolution of galaxies.
This Chapter looks at the present-day gas metallicities of outer spiral disks, as derived from the emission line analysis of H ii region spectra, excluding older chemical abundance tracers, such as planetary nebulae and stars, except for a few notable exceptions. The connections between the chemical abundances of the outer disks thus derived and of the circumgalactic medium, probed by resonance absorption lines in the UV, for example in damped Lyman α systems, is covered elsewhere in this volume (see the review by Chen, this volume).
A non-secondary aspect of chemical abundance work in nearby and far-away star-forming galaxies concerns the methodology employed in the measurement of nebular (ionized gas) abundances. Therefore, Sect. 2 provides a brief overview of the difficulties and the techniques used. The subsequent sections provide details on the work carried out in a variety of nearby systems (Sect. 3 and 4), building the framework for interpreting the observed chemical abundance properties (Sect. 5). A concise summary concludes the Chapter.