  We review the properties of turbulent molecular clouds (MCs), focusing on the
physical processes that influence star formation (SF). MC formation appears to
occur during large-scale compression of the diffuse ISM driven by supernovae,
magnetorotational instability, or gravitational instability in galactic disks
of stars and gas. The compressions generate turbulence that can accelerate
molecule production and produce the observed morphology. We then review the
properties of MC turbulence, including density enhancements observed as clumps
and cores, magnetic field structure, driving scales, the relation to observed
scaling relations, and the interaction with gas thermodynamics. We argue that
MC cores are dynamical, not quasistatic, objects with relatively short
lifetimes not exceeding a few megayears. We review their morphology, magnetic
fields, density and velocity profiles, and virial budget. Next, we discuss how
MC turbulence controls SF. On global scales turbulence prevents monolithic
collapse of the clouds; on small scales it promotes local collapse. We discuss
its effects on the SF efficiency, and critically examine the possible relation
between the clump mass distribution and the initial mass function, and then
turn to the redistribution of angular momentum during collapse and how it
determines the multiplicity of stellar systems. Finally, we discuss the
importance of dynamical interactions between protostars in dense clusters, and
the effect of the ionization and winds from those protostars on the surrounding
cloud. We conclude that the interaction of self-gravity and turbulence controls
MC formation and behavior, as well as the core and star formation processes
within them.
