  I present an overview of the hierarchy of structures existing in the
interstellar medium (ISM) and the possible mechanisms that cause the
fragmentation of one level into the next, with the formation of stars as its
last step. Within this framework, I then give an overview of the contributions
to this session. Numerical work addresses, at the largest scales, the shaping
and formation of structures in the ISM through turbulence driven by stellar
energy injection, and the resulting star formation rate as a function of mean
density. At the scales of molecular clouds, results comparing observational and
numerical data on the density and velocity structure of turbulence-produced
cores, as well as their mass spectra, are summarized, together with existing
theories of core and star formation controlled by the turbulence.
Observationally, an attempt to discriminate between the standard and turbulent
models of star formation is presented, finding inconclusive results, but
suggesting that both turbulence and the magnetic field are dynamically
important in molecular clouds and their cores. Finally, various determinations
of the magnetic field strength and geometry are also presented.
