  We briefly describe the three existing scenarios for forming massive stars
and emphasize that the arguments often used to reject the accretion scenario
for massive stars are misleading. It is usually not accounted for the fact that
the turbulent pressure associated to large turbulent velocities in clouds
necessarily imply relatively high accretion rates for massive stars. We show
the basic difference between the formation of low and high mass stars based on
the values of the free fall time and of the Kelvin-Helmoltz timescale, and
define the concept of birthline for massive stars.
  Due to D-burning, the radius and location of the birthline in the HR diagram,
as well as the lifetimes are very sensitive to the accretion rate \dot M_accr.
If a form \dot M_accr=A (M/M_sun)^phi is adopted, the observations in the HR
diagram and the lifetimes support a value of A=10^{-5} M_sun/yr and a value of
phi>1. Remarkably, such a law is consistent with the relation found by
Churchwell (1998) and Henning et al. (2000) between the outflow rates and the
luminosities of ultra--compact HII regions, if we assume that a fraction 0.15
to 0.3 of the global inflow is accreted. The above relation implies high \dot
M_accr=10^{-3} M_sun/yr for the most massive stars. The physical possibility of
such high \dot M_accr is supported by current numerical models.
  Finally, we give simple analytical arguments in favour of the growth of \dot
M_accr with the already accreted mass. We also suggest that due to Bondi-Hoyle
accretion, the formation of binary stars is largely favoured among massive
stars in the accretion scenario.
