Here I present an overview of the patterns of metallicity and element abundance ratios observed in spiral and irregular galaxies. I will discuss the results for both types of galaxies rather than separately; many aspects can be discussed for the combined groups, although there are a number of differences that could constitute the topic of an entire conference alone.
The observational data to be discussed represent a highly selected sample of abundances, gas masses, and stellar photometry from sources too numerous to mention here. (If you recognize your data in the following plots, feel free to take credit.) I will employ abundances derived almost exclusively from H II region spectra, since they contribute the largest set of abundance data for spirals and irregulars in the local universe.
4.1. Gas and Stellar Masses
The ultimate goal of any chemical evolution model is to account for the global and local metallicity within a galaxy, the gas and stellar mass distributions, and the stellar luminosity self-consistently. Thus, any discussion of abundances and chemical evolution should include a few words about observational determinations of gas and stellar masses and mass surface densities.