EXPRMNTL, also known as the Knokke Experimental Film Festival and Festival du Film Expérimental de Knokke-le-Zoute is said to be the most important event ever organized for avant-garde and experimental film.
It started as a showcase for experimental film in 1947 as part of the Brussels Experimental Film Festival, followed by a second more specific experimental film festival in 1958 as part of the World Exhibition in Brussels. The festival was later held under the EXPRMNTL moniker in 1963 (EXPRMNTL 3), 1967 (EXPRMNTL 4) and 1974 (EXPRMNTL 5).
The festival, organized in Knokke-le-Zoute, Belgium, in an empty casino between Christmas and New Year, was a key place for experimental film in the critical period of its development, and almost the only meeting point for avant-garde filmmakers for years, the place where experimental movements connected.
It was conceived and curated by Jacques Ledoux and the Cinémathèque royale de Belgique. Ledoux was, at the time, conservator of the Belgian National Film Archive (also known as the Cinémathèque) in Brussels.
The attendees have included Jonas Mekas, Peter Kubelka, Yoko Ono, Jean-Jacques Lebel, Jack Smith, Martin Scorsese, Holger Meins, Pierre Vermeylen, as well as Marguerite Duras, Marcel Broodthaers, Roman Polanski, Mauricio Kagel, Michael Snow, Hugo Claus, Pierre Clémenti, Martial Raysse, Nam June Paik, P. Adams Sitney, Jean-Luc Godard and many others.
German film students and sympathisers demonstrating against the festival, 31 December 1967. Photograph by Virginia Leirens. 
A photograph taken by Ed van der Elsken during the election of Miss Expérimentation. On the left, Jean-Jacques Lebel and Harun Farocki. Sitting at a table are the members of the selection jury. Standing on that table, a naked candidate to the title of Miss Expérimentation. On the wall, a detail of René Magritte's fresco Le Domaine Enchanté. Published in Skoop, 1968. 
The second edition concurred with the World Exhibition in Brussels; Kenneth Anger, Jonas Mekas, Robert Breer, Stan Brakhage, Peter Kubelka, Jan Lenica and Walerian Borowczyk attended. The first prizes were won by Dom and Touch of Evil. 137 films from 29 countries including 7 entries from Poland participated in the competition. The shortlist of the most prestigious artists consists of (in alphabetical order): Kenneth Anger, Stan Brakhage, Robert Breer, Marcel Broodthaers, Mary Ellen Bute, Shirley Clarke, Carmen d’Avino, Maya Deren, Georges Franju, Abel Gance, Claude Goretta, Yoram Gross, John Hubley, Lewis Jacobs, Larry Jordan, Nelly Kaplan, Peter Kubelka, Len Lye, Willard Maas, Marie Menken, Jean Mitry, Ernest Pintoff, François Reichenbach, Ken Russell, Alain Tanner, Stan Vanderbeek, Agnes Varda, John Whitney.
The weeklong third edition began on Christmas Day of 1963. It would become most famous for a riotous partial screening of Jack Smith‘s Flaming Creatures, an incident that has been well-documented and discussed ever since. More.
Of the 170 films shown at the festival, five British entries were included in the competition and John Latham’s celebrated Speak (1962) was a notable omission. However a series of films by Stephen Dwoskin went on to win the Solvay Prize. Michael Snow’s film Wavelength (1967) won the festival’s Grand Prize: a 45-minute zoom across a New York loft apartment, interrupted at various points by changes in the film stock and lens filters, which inspired much of the ‘Structuralist’ filmmaking in the late 1960s and early 1970s.
The festival also saw the stirrings of a potential European Filmmakers Co-op which was to be named ‘Europ’ and, although a meeting took place in Munich in the year following the festival, the representatives of the various European groups could not agree to work together.
The festival had an influential effect on the London Scene, particularly on members of the London Filmmakers Co-op, especially David Curtis, who was programming for The Arts Lab at the time and reported on the festival for International Times. 
In the historical context of the Casino, the programme featuring film screenings, live performances, talks and an exhibition of rare documents related to EXPRMNTL discussing the history and importance of the festival took place on 3 May 2009 in the context of the Internationaal Fotofestival Knokke-Heist, 2009. Curated by Xavier Garcia Bardon. Coordinated by Kevin Decoster.  
- Gerda Johanna Cammaer, "EXPRMNTL 3 / Knokke-le-Zoute 1963. Flaming Creatures, Raving Features".
- Revue Belge du Cinéma, Special Issue: "EXPRMNTL: Festival Hors Normes, Knokke 1963 1967 1974", No. 43, (December 2002), (59).
- Mister Trippy, "People let’s freak out!", 2009.
- María Palacios Cruz, "En Belgique : Un Cinéma des limites", L'Art Même, No. 42, 2009, pp 8-9. (French)
- Xavier Garcia Bardon, "EXPRMNTL: An Expanded Festival. Programming and Polemics at EXPRMNTL 4, Knokke-le-Zoute, 1967", Cinema Comparat/ive Cinema, Vol. 1, No. 2, 2013, pp 53-64.
- Mike Everleth, "EXPRMNTL 3: 1963 Recap", Underground Film Journal, 1 Oct 2017.