Art or vandalism?

It happens very often to see huge murals around the cities all around Italy. Art or degradation? Adornment or mess? A mural can be one or the other, depending on where, how and why it is drawn. Which is the line between art and vandalism? What do you think about this? Do you know places where art is expressed on the walls with the approval of the government or where there are fine works, but which are true acts of vandalism?