  Gas processes affecting star formation are reviewed with an emphasis on
gravitational and magnetic instabilities as a source of turbulence.
Gravitational instabilities are pervasive in a multi-phase medium, even for
sub-threshold column densities, suggesting that only an ISM with a pure-warm
phase can stop star formation. The instabilities generate turbulence, and this
turbulence influences the structure and timing of star formation through its
effect on the gas distribution and density. The final trigger for star
formation is usually direct compression by another star or cluster. The star
formation rate is apparently independent of the detailed mechanisms for star
formation, and determined primarily by the total mass of gas in a dense form.
If the density distribution function is a log-normal, as suggested by
turbulence simulations, then this dense gas mass can be calculated and the star
formation rate determined from first principles. The results suggest that only
10**(-4) of the ISM mass actively participates in the star formation process
and that this fraction does so because its density is larger than 10**5 cm-3,
at which point several key processes affecting dynamical equilibrium begin to
break down.
