Although intervention by its nature carries an implication of subversion, it is now accepted as a legitimate form of art and is often executed with the endorsement of those in positions of authority over the artwork, audience, or venue/space to be intervened in. However, unendorsed (i.e. illicit) interventions are common and lead to debate as to the distinction between art and vandalism.[1] By definition it is a challenge, or at the very least a comment, related to the earlier work or the theme of that work, or to the expectations of a particular audience, and more likely to fulfil that function to its full potential when it is unilateral, although in these instances, it is almost certain that it will be viewed by authorities as unwelcome, if not vandalism, and not art.