  Whether or not molecular clouds and embedded cloud fragments are stable
against collapse is of utmost importance for the study of the star formation
process. Only "supercritical" cloud fragments are able to collapse and form
stars. The virial parameter, alpha=M_vir/M, which compares the virial to the
actual mass, provides one way to gauge stability against collapse.
Supercritical cloud fragments are characterized by alpha<2, as indicated by a
comprehensive stability analysis considering perturbations in pressure and
density gradients. Past research has suggested that virial parameters alpha>2
prevail in clouds. This would suggest that collapse towards star formation is a
gradual and relatively slow process, and that magnetic fields are not needed to
explain the observed cloud structure. Here, we review a range of very recent
observational studies that derive virial parameters <<2 and compile a catalogue
of 1325 virial parameter estimates. Low values of alpha are in particular
observed for regions of high mass star formation (HMSF). These observations may
argue for a more rapid and violent evolution during collapse. This would enable
"competitive accretion" in HMSF, constrain some models of "monolithic
collapse", and might explain the absence of high--mass starless cores.
Alternatively, the data could point at the presence of significant magnetic
fields ~1 mG at high gas densities. We examine to what extent the derived
observational properties might be biased by observational or theoretical
uncertainties. For a wide range of reasonable parameters, our conclusions
appear to be robust with respect to such biases.
