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

Low Virial Parameters in Molecular Clouds: Implications for High Mass Star Formation and Magnetic Fields