  Observations of magnetic field strengths imply that molecular cloud fragments
are individually close to being in a magnetically critical state, even though
both magnetic field and column density measurements range over two orders of
magnitude. The turbulent pressure also approximately balances the
self-gravitational pressure. These results together mean that the
one-dimensional velocity dispersion $\sigv$ is proportional to the mean \Alf
speed of a cloud $\va$. Global models of MHD turbulence in a molecular cloud
show that this correlation is naturally satisfied for a range of different
driving strengths of the turbulence. For example, an increase of turbulent
driving causes a cloud expansion which also increases $\va$. Clouds are in a
time averaged balance but exhibit large oscillatory motions, particularly in
their outer rarefied regions. We also discuss models of gravitational
fragmentation in a sheet-like region in which turbulence has already
dissipated, including the effects of magnetic fields and ion-neutral friction.
Clouds with near-critical mass-to-flux ratios lead to subsonic infall within
cores, consistent with some recent observations of motions in starless cores.
Conversely, significantly supercritical clouds are expected to produce extended
supersonic infall.
