Graffiti has always existed to a certain extent, but in the 1970s, when New York City was dirty and dangerous, it flourished. Clandestine art sprang up like weeds in (and on) subway cars. The Bronx, which was rising from the ashes, was a favorite canvas. Artists were soon embellishing entire cars and building facades with de facto murals. Around the same time, in 1973, Clive Campbell (stage name: D.J. Kool Herc) and his sister, Cindy, put on a “Back to School Jam” party in the recreation room of their Bronx apartment building. That rec room is now known as the birthplace of hip-hop. As hip-hop proliferated, graffiti art took over the Americas, Europe, and Asia. Curated by the graffiti historian Roger Gastman, this exhibition sees the takeover of three floors at Saatchi by 100 artists, making it one of the biggest street-art shows in history. —E.C.