  Observations of both star-forming regions and young, gas-free stellar
associations indicate that most nearby molecular clouds form stars only over a
short time span before dispersal; large-scale flows in the diffuse interstellar
medium have the potential for forming clouds sufficiently rapidly, and for
producing stellar populations with ages much less than the lateral crossing
times of their host molecular clouds. We identify four important factors for
understanding rapid star formation and short cloud lifetimes. First, much of
the accumulation and dispersal of clouds near the solar circle might occur in
the atomic phase; only the high-density portion of a cloud's lifecycle is spent
in the molecular phase, thus helping to limit molecular cloud ``lifetimes''.
Second, once a cloud achieves a high enough column density to form $\h2$ and
CO, gravitational forces become larger than typical interstellar pressure
forces; thus star formation can follow rapidly upon molecular gas formation and
turbulent dissipation in limited areas of each cloud complex. Third, typical
magnetic fields are not strong enough to prevent rapid cloud formation and
gravitational collapse. Fourth, rapid dispersal of gas by newly-formed stars,
and reduction of shielding by a small expansion of the cloud after the first
events of star formation, might limit the length of the star formation epoch
and the lifetime of a cloud in its molecular state. This picture emphasizes the
importance of large-scale boundary conditions for understanding molecular cloud
formation, and implies that star formation is a highly dynamic, rather than
quasi-static, process.
