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.

13 pages, 1 jpg figure, 1 eps figure. For publication in a proceedings issue of Astrophysics & Space Science recording the conference "From Observations to Self-Consistent Modeling of the Interstellar Medium", held in Porto, Portugal