  We review recent high spatial resolution millimeter continuum and spectral
line observations of (proto-)cluster regions. These observations reveal that
the mass distribution of prestellar cores is consistent with the initial mass
function for field stars suggesting that the IMF is connected to the molecular
clouds structure or the cloud fragmentation processes, rather than the details
of the star formation mechanism. We discuss the evidence for at-birth
subclustering independently obtained for two separate protoclusters. The
presence of subclustering within coherent subclumps suggests that the
fragmentation process is hierarchical and that young stellar clusters are
assembled by independent substructures. The local (proto-)stellar density and
star formation efficiency within the subclusters are much higher than the
average values for the entire star forming cloud. Unfortunately, current
observations are sparse and limited to the nearest star forming regions, for
spatial resolution and sensitivity considerations, the future millimeter wave
arrays, especially ALMA, will allow to expand considerably the sample of
observed regions.
