In cinema, a making-of, also known as behind-the-scenes, the set or on the set is a documentary film that features the production of a film or television program. This is often referred to as the EPK (electronic press kit) video, due to its main usage as a promotional tool, either concurrent with theatrical release or as a bonus feature for the film's DVD or Blu-ray release.
Feature length documentaries on the making of other films has become a film genre in its own right. The making-of film may ultimately be more important than the film itself. In the case of Lost in La Mancha, the making-of video ended up documenting the collapse and abandonment of the feature film it was covering (The Man Who Killed Don Quixote), and ultimately the making-of video was the only part of the production to see a commercial release as a theatrical documentary feature. 
Shorter making-of documentaries are often used as a bonus on DVDs, as it offers more insight into the film, how it was made, and to credit the film crew. Occasionally, some films have included a "making of the making-of" as a joke. The making-of is also often released for TV as a part of the promotion of the film.