Progress in understanding the chemical evolution of the Milky Way and other galaxies has relied on an increasing wealth of observations and a variety of theoretical inputs. Modern ideas depart from the concept of a monolithic collapse of the protogalaxy (Eggen, Lynden-Bell, and Sandage 1962), embracing mergers, cannibalism, infall, and radial flows in the disk. This complicated set of processes must be constrained by a commensurate set of observations. For the Milky Way, stellar abundances as a function of age and kinematics are increasingly available. Evidence for mergers in the history of the Galaxy can be found in the kinematics of "star streams" (Majewski 1999). Abundance measurements of individual stars in external galaxies are made possible by large telescopes and modern spectrographs (e.g., Venn et al. 2001).
Ionized nebulae remain the best way to measure the abundances of many elements in the interstellar gas of galaxies. With sensitive detectors on large telescopes, this work will increasingly involve objects at significant redshifts and in a variety of environments, and precise measurements of local abundance fluctuations on various length scales. Ionized nebulae will continue to play an important role in the description of the chemical evolution of the universe from its youth to the present.
Acknowledgments
I am indebted to Don Garnett, Chip Kobulnicky, Bernard Pagel, Evan Skillman, and Chris Sneden for helpful discussions.