Ionized nebulae have played an important role in the measurement of interstellar abundances in our own galaxy and others while also serving as a laboratories for atomic physics. Early nebular studies focused on planetary nebulae and H II regions in our galaxy. These showed abundances roughly similar to those in the sun, but with significant differences depending on the element, location in the Galaxy, and the stellar population involved. The advent of sensitive detectors and the increasing availability of large telescopes made possible the systematic study of giant extragalactic H II regions (GEHRs), planetary nebulae, and supernova remnants in external galaxies. This work revealed patterns of variation of chemical composition with position in a galaxy, and from galaxy to galaxy. Today we know that chemical abundances decrease outward across the disks of galaxies, and that they increase with increasing galactic luminosity. These trends are qualitatively echoed in the abundances of stars in elliptical galaxies. The relative abundances of the elements also show systematic trends. There are indications that the cluster environment affects the chemical evolution of spiral galaxies. Observations of QSO emission lines suggest a metal rich environment in galactic nuclei even at high redshift. Studies of QSO absorption lines probe the interstellar medium of galaxies at early times. These results provide a rich set of constraints for theoretical models. For a detailed discussion of abundances in spiral and elliptical galaxies, the reader is referred to the excellent review by Henry & Worthey (1999, ``HW''). Observational results and theoretical foundations are discussed in the classic review by Tinsley (1980).