Theory of Feedback in Clusters and Molecular Cloud Turbulence
  I review recent numerical and analytical work on the feedback from both low-
and high-mass cluster stars into their gasoeus environment. The main
conclusions are that i) outflow driving appears capable of maintaing the
turbulence in parsec-sized clumps and retarding their collapse from the
free-fall rate, although there exist regions within molecular clouds, and even
some examples of whole clouds, which are not actively forming stars, yet are
just as turbulent, so that a more universal turbulence-driving mechanism is
needed; ii) outflow-driven turbulence exhibits specific spectral features that
can be tested observationally; iii) feedback plays an important role in
reducing the star formation rate; iv) nevertheless, numerical simulations
suggest that feedback cannot completely prevent a net contracting motion of
clouds and clumps. Therefore, an appealing source for driving the turbulence
everywhere in GMCs is the accretion from the environment, at all scales. In
this case, feedback's most important role may be to prevent a fraction of the
gas nearest to newly formed stars from actually reaching them, thus reducing
the star formation efficiency.
