The Role of HI in the Star-Formation Processes in Spiral Disks

We discuss recent work on massive star-formation efficiency parameters on 1 kpc scales in grand-design spiral galaxies. The fact that in two galaxies studies in detail, M51 and NGC 4321, the massive star-formation efficiency in the arms is larger than that outside the arms can be seen as evidence for large-scale triggering of the star formation in the arms, probably induced by a density wave system. We discuss several possible systematic effects, but infer that the conclusions on triggering are sound.