After a brief introduction into the evolution of spiral galaxies, the observational evidence for temporal and spatial abundance variations across the galactic disk is discussed. The theoretical framework of chemical evolution models is summarised, followed by a critical discussion of available constraints. Solutions applying to the special cases of the solar vicinity and the galactic abundance gradients are presented. We summarize evidence for a bimodal nature of star formation, and emphasize that if in regions and periods of high star formation activity the formation of low-mass stars is suppressed, a self-consistent model of star formation and chemical evolution of the Milky Way disk can be presented.