  We describe an overall picture of galactic-scale star formation. Recent
high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with
cooling/heating and thermal conduction have shown that the formation of
molecular clouds requires multiple episodes of supersonic compression. This
finding enables us to create a scenario in which molecular clouds form in
interacting shells or bubbles on a galactic scale. First we estimate the
ensemble-averaged growth rate of molecular clouds over a timescale larger than
a million years. Next we perform radiation hydrodynamics simulations to
evaluate the destruction rate of magnetized molecular clouds by the stellar FUV
radiation. We also investigate the resultant star formation efficiency within a
cloud which amounts to a low value (a few percent) if we adopt the power-law
exponent -2.5 for the mass distribution of stars in the cloud. We finally
describe the time evolution of the mass function of molecular clouds over a
long timescale (>1Myr) and discuss the steady state exponent of the power-law
slope in various environments.
