In our quest to elucidate the origin of the universe and the formation of galaxies, particularly that of the Milky Way in which we live, astounding progress has been made in recent years through observational and theoretical studies. Not only have gigantic surveys covering a large fraction of the sky brought statistics enlightening evolutionary paths of galaxies, but powerful instruments, such as radio interferometers and ground- and space-based optical/ infrared telescopes, have been able to map individual objects with high sensitivity and spatial resolution. Yet we do not fully understand the physics behind the observational results, and a number of unsolved problems need to be discussed, such as: What.
