Invited review abstract

The properties of the earliest phases of the formation of high-mass stars should clearly discriminate the different theoretical views which may explain the origin of high-mass stars. These phases (pre-collapse cores and collapsing proto-stars) are however extremely difficult to recognize. They are short and the high-mass proto-stars are thus rare and distant. The high-mass proto-stars are also embedded in crowded and dense proto-clusters and could be detectable only in the far-infrared and sub-millimeter ranges. Finally in competitive accretion scenarii, the high-mass precursors could even be virtually impossible to recognize since they may not be associated, at any stages, with any high-mass cores or envelopes.
I will review the very recent new observational results on the best candidates so far to be true precursors of OB stars with a focus on nearby regions, and on the efforts at the scale of the Galaxy to access large, representative samples of high-mass star formation sites which will be soon investigated with ALMA to search for individual collapsing objects forming high-mass stars at large Galactic distances.