  The evolution of galaxies results from a combination of internal and external
processes. The star formation is an internal process transforming cold and
dense cores of molecular clouds to stars. It may be triggered internally by
expanding shells, or externally, e. g. by galaxy collisions. The gas accretion
and galaxy merging events are external contributors to galaxy evolution. They
compete with another internal process of galaxy evolution, which is the secular
evolution redistributing the mass and angular momentum inside of galaxies as a
consequence of bar and spiral arms formation. As a nearby example of gas
accretion we mention the interacting system Milky Way - Large and Small
Magellanic Clouds. Stripping of ISM in galaxy clusters is reviewed as another
example of interaction of galaxies with their environment.
