  Star formation is enhanced in spiral arms because of a combination of orbit
crowding, cloud collisions, and gravitational instabilities. The characteristic
mass for the instability is 10^7 Msun in gas and 10^5 Msun in stars, and the
morphology is the familiar beads on a string with 1-2 kpc separation. Similar
instabilities occur in resonance rings and tidal tails. Sequential triggering
from stellar pressure occurs in two ways. For short times and near distances,
it occurs in the bright rims and dense knots that lag behind during cloud
dispersal. For long times, it occurs in swept-up shells and along the periphery
of cleared regions. The first case should be common but difficult to
disentangle from independent star formation in the same cloud. The second case
has a causality condition and a collapse condition and is often easy to
recognize. Turbulent triggering produces a hierarchy of dense cloudy structure
and an associated hierarchy of young star positions. There should also be a
correlation between the duration of star formation and the size of the region
that is analogous to the size-linewidth relation in the gas. The cosmological
context is provided by observations of star formation in high redshift
galaxies. Sequential and turbulent triggering is not yet observable, but
gravitational instabilities are, and they show a scale up from local
instabilities by a factor of ~3 in size and ~100 in mass. This is most easily
explained as the result of an increase in the ISM turbulent speed by a factor
of ~5. In the clumpiest galaxies at high redshift, the clumps are so large that
they should interact with each other and merge in the center, where they form
or contribute to the bulge.
