During the month of September, my focus will be on gangs. Mostly street gangs, such as the Bloods, the Crips, the Mexican Mafia, the Black Disciples, Gangster Disciples and many others. Gangs seem to be a part of every state and every large city. They are in places you don’t expect them to be. It has become an overwhelming problem that needs to be addressed. As with the mafia, it used to be hard to convict them and keep them in prison. But since RICO came about, it has made it much easier.
According to the FBI:
About 30,000 violent street gangs, motorcycle gangs, and prison gangs with approximately 800,000 members operate in the U.S. today.
Examples of gang graffiti
Know Gangs (a very good site with alot of information)
Street Gangs
FBI: Violent Gangs
Chicago Gangs
Wikipedia: Mara Salvatrucha
Gang Life In Los Angeles:
The East Side Story
Bonnie's Blog of Crime 
I heard they in the army too
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Those are not all gang graffiti. The colourful ones are done by taggers, not gang members. Get your facts straight.
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The mexican mafia are a prison gang, not a street gang. There are street gangs aligned with the MM but they are not actually mexican mafia.
Also the 1st picture is definitely not ‘gang graffiti’. These are murals, most probably painted legally and definitely not painted by gang members. The 2nd picture is a better example of gang writing.
Gang graffiti is usually limited to a row (roll call) of names & tags written in a simple, or occasionally old english style script, with a single colour.
Apart from that it’s an interesting site ya got here
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Whoops… Just saw someone already pointed that out
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