  We discuss the mechanism of cluster formation in hierarchically collapsing
molecular clouds. Recent evidence, both observational and numerical, suggests
that molecular clouds (MCs) may be undergoing global, hierarchical
gravitational collapse. The "hierarchical" regime consists of small-scale
collapses within larger-scale ones. The latter implies that the star formation
rate increases systematically during the early stages of evolution, and occurs
via filamentary flows onto "hubs" of higher density, mass, and velocity
dispersion, and culminates a few Myr after than the small-scale collapses have
started to form stars. In turn, the small-scale collapses occur in clumps
embedded in the filaments, and are themselves falling into the larger potential
well of the still-ongoing large-scale collapse. The stars formed in the early,
small-scale collapses share the infall motion of their parent clumps towards
the larger potential trough, so that the filaments feed both gaseous and
stellar material to the hubs. This leads to the presence of older stars in a
region where new protostars are still forming, to a scale-free or fractal
structure of the clusters, in which each unit is composed of smaller-scale
ones, and to the eventual merging of the subunits, explaining the observed
structural features of open clusters.
