  We discuss the role of turbulence in cloud and star formation, as observed in
numerical simulations of the interstellar medium. Turbulent compression at the
interfaces of colliding gas streams is responsible for the formation of
intermediate ($\simlt 100$ pc) and small clouds (a few tens of pc), although
the smallest clouds can also form from fragmentation of expanding shells around
stellar heating centers. The largest cloud complexes (several hundred pc) seem
to form by slow, gravitational instability-driven merging of individual clouds,
which can actually be described as a large-scale tendency towards
homogenization of the flow due to gravity rather than cloud collisions. These
mechanisms operate as well in the presence of a magnetic field and rotation,
although slight variations on the compressibility and cloud morphology are
present which depend on the strength and topology of the field. In summary, the
role of turbulence in the life-cycle of clouds appears to be twofold:
small-scale modes contribute to cloud support, while large-scale modes can both
form or disrupt clouds.
