Wild, free and ephemeral – street art and graffiti are a worldwide phenomenon.From Brassaï, who led graffiti into mainstream art around 1960, via the Sprayer of Zurich in the 1970s to the first Banksy works in the UK – Illegal tells a prehistory of street art and graffiti. Hardly any of the works of this short-lived genre still exist today.They were also often documented illegally and under adverse conditions.Illustrated here are key works and rarities that have never been shown.All were created illegally, i.e. without permission, directly for an audience on the street – not for museum contexts. “People say graffiti is ugly, irresponsible and childish.But that’s only if it’s done properly” [Banksy]. Long-standing picture research has condensed a selection of works by some 100 artists from over a dozen countries and their references to pop music, avant-garde art and literature.We discuss why these artists in particular were significant trendsetters for street art and graffiti before Banksy.