  The impact of radiation pressure on the dynamics of the gas in the vicinity
of young stellar clusters is thoroughly discussed. The radiation over the
thermal/ram pressure ratio time evolution is calculated explicitely and the
crucial role of the cluster mechanical power and of the strong time evolution
of the ionizing photon flux and of the bolometric luminosity of the exciting
cluster is stressed. It is shown that radiation has only a narrow window of
opportunity to dominate the wind-driven shell dynamics. This may occur only at
early stages of the bubble evolution and if the shell expands into a dusty
and/or a very dense proto-cluster medium. The impact of radiation pressure on
the wind-driven shell becomes always negligible after about 3 Myr. Finally, the
wind-driven model results allow one to compare the model predictions with the
distribution of thermal pressure derived from X-ray observations. The shape of
the thermal pressure profile allows then to distinguish between the energy and
the momentum dominated regimes of expansion and thus conclude whether radiative
losses of energy or the leakage of hot gas from the bubble interior have been
significant during the bubble evolution.
