"Humans have been expressing themselves by scrawling on walls since the earliest people lived in caves. But it wasn’t until the 1970s that we started taking our messages to the walls, trains and sidewalks of urban environments around the world. The graffiti movement, seen by some as mere vandalism, actually encompasses some impressive and talented artists. They’re attracted by the freedom that graffiti provides; the ability to simply make something without constraints. Some are also involved in more widely-accepted art forums, such as gallery shows, but many choose to remain anonymous and shrouded in a world of secrecy, pseudonyms, and spray paint."
WebUrbanist just made their selection "10 of the best Graffiti Artists" over here