Fifty years ago the Happening was what was really happening, on the international visual art scene.
Instead of people visiting galleries to look at art objects, audiences became involved in a series of events that made no distinction between observer and observed, and sometimes didn't seem to have a beginning, middle or end.
Happenings paved the way for perfomance art and live art.
And the 60s wouldn't have been half as groovy without them.
Early Happenings, like Allan Kaprow's 18 Happenings in 6 Parts, Claes Oldenburg's Snapshots From A City and Al Hansens' Requiem For W.C.
Fields, inspired a generation of young artists, and grabbed the attention of the public.
In this programme, Bob Dickinson re-imagines the Happening with contemporary artists and writers including RoseLee Goldberg, Ilana Mitchell, Andrea Cusmano, and Lorenzo Fusi.
Dickinson also investigates the way Happenings travelled around the world, to Asia and Europe, thanks to Nam June Paik in Korea and Japan, Adrian Henri in Liverpool, the Viennese Aktionists, Joseph Beuys in Germany, and Yves Klein in France.
As Happenings became part of popular culture, they became increasingly spectacular, and in 1967 Britain's biggest ever example of the activity was staged at Alexandra Palace in London - the 24 Hour Technicolour Dream.
It was a high-point, in more ways than one, for the hippie counterculture, but it also marked the point when Happenings stopped being art.
We meet an eye-witness.
Now, half a century after Happenings transformed the art scene, some contemporary artists, such as Tanya Brughera, from Cuba, are revisiting and reviving several of Allan Kaprow's classic Happenings for a new audience - and we find out what happens when she tries them out on the streets and shops of Liverpool.
The Happening, that essential feature of 60s bohemia, is back.
Far out, or too late?