Some problems and new directions for theoretical work on large scale star formation are discussed. The role of density waves in the context of star formation seems to be primarily one of organization: The waves place most of the gas in the arms, so most of the star formation is in the arms too. The waves also promote the coagulation of small clouds into large cloud complexes, or superclouds, by what appears to be a combination of collisional agglomeration and large-scale gravitational instabilities. Special regions where density waves do trigger a true excess of star formation are discussed, and possible reasons for the difference between these triggering waves and the more common, organizing, waves are mentioned. Our understanding of star formation is limited by our lack of knowledge of the macroscopic equation of state for clumpy, magnetic interstellar matter. © 1985 IOP Publication Ltd.