Although only small by number massive stars influence the evolution of galaxies most efficiently. Their short lifetimes in combination with enormous amounts of power by radiative and kinetic energy and furthermore the release of substantial fractions of heavy elements make them the most important drivers of the thermal and chemical galaxy evolution. Most impressively, their final explosions as type II supernovae energize the Interstellar Medium (ISM) to an extent that it can become unbound to galaxies and is lost as a galactic wind, by this, feeding the intergalactic medium with hot metal-rich gas or storing it as galactic halo gas and, as an extreme, leading to gas-free systems or even more extremely to the disruption of low-mass galaxies. In addtion, already during their lives massive stars are highly influencial to their surroundings by means of their ionizing radiation and strong stellar winds. In total, over the lifetime of a massive star his energy release can exceed the supernova explosion energy. This becomes visible e.g. by a rapid clearance of star forming regions from their native cloud and e.g. by the production of a diffusely ionized component of the ISM. Despite their relevance, lots of questions are still unexplored and require to be addressed, like e.g.the following: How much energy from massive stars is really deposited in the ISM? Is the release of heavy elements from peeled-off burning shells in the Wolf-bRayet stage already contributing to the element enhancement of the warm photo-ionized HII regions from which the abundances of the natal ISM are determined? To what extent is the inhomogeneity of the ISM structures by dynamical processes? Do we interpret the observations of HII regions with correct models? These and further questions will be addressed in the presentation.