  Molecular clouds (MC) are the densest and coldest component of the
interstellar gas, and the sites of star formation. They are also turbulent and
fractal and their masses and sizes span several orders of magnitude. It is also
generally believed that they are close to Virial equilibrium (VE). Since this
statement has been questioned by a number of authors, with important
implications on molecular clouds' lifetimes, we will review this subject within
the context of a turbulent ISM. In this framework, there is significant
numerical evidence that MCs are not in VE, that there is a strong exchange of
mass, momentum and energy between clouds and their surrounding medium, and that
it is difficult (if not impossible) to form quasistatic cores inside MCs,
suggesting that they must be transient, short-lived phenomena. Thus, their
formation and disruption must be primarily dynamical, and probably not due to
just a single mechanism, but rather to the combination of several processes.
This picture seems consistent with recent estimates of ages of stars in the
solar neighborhood.
