The structure and dynamics of the interstellar medium (ISM) are strongly affected by the action of massive stars. They deposit in the ISM a huge number of ionizing photons and transfer to it vast amounts of mechanical energy via their powerful stellar winds. As a consequence, massive stars create what is known as an interstellar bubble, i.e., a minimum in the gas distribution characterized by a low volume density and a high temperature, surrounded by an expanding envelope (Weaver et al. 1977). The star should be seen projected onto, or close to, the centre of the H i minimum.