Improvisation is the act of making something up as you go along. This term is usually used in the context of music or theater.

In music, jazz and Bluegrass are well-known for using improvisation. It features in many kinds of traditional music, including flamenco, Carnatic music, Pygmy and other African music, and was once an important element in classical music (see cadenza and figured bass). Improvisation can be structured, with certain rules constraining the improvisation (for example, "make up a song about bicycles", "use these chord changes", and so on), or can have no such constraints (free improvisation). A growing number of contemporary composers are requiring a greater degree of improvisation in their music, including Pauline Oliveros, Terry Riley, and Christian Wolff.

Most aspiring actors do a lot of improv. It is a staple of drama and theater classes at most colleges and high schools. According to the dominant acting theories of Konstantin Stanislavski, an actor improvising a scene must be trusting his own instincts. According to Stanislavski (see method acting), an actor must use his own instincts to define a characters response to internal and external stimuli. Through improvising, an actor can learn to trust his instincts instead of using mugging and indicating to broadcast his motives. Improv is also useful in its focus on concentration. Obviously, in an environment in which anything is allowed to happen, the actors must be capable of keeping their concentration throughout, even in difficult and stressful circumstances. Concentration is a staple of acting classes and workshops; it is vital than an actor be capable of concentrating on the scene or action at hand.

Improvisation is also performed as an art form itself in theatres around the world, sometimes with dramatic intent but more often in comedic form (the most famous is Chicago's Second City). Through the work of great minds as Viola Spolin, Paul Sills, Del Close and Keith Johnstone, actors improvise scenes which are often funny with amazing character work and recognizable human behavior.

In the 1990s, a TV show called Whose Line Is It Anyway popularized comedic improvisation. The original version was British, but it was later revived and popularized in the United States with Drew Carey as a host.

Some Role-playing games (tabletop games, live action games, MUDs and some MMORPG computer games) often involve a casual form of improvisational acting. (See gamemaster for an example.) A player's character may be pre-defined, with game statistics and a history, but the character's response to game events and to other players is improvised. Some players are more interested in the depth of the "acting" than others; some are purely combat and game-mechanic oriented, while others enjoy elaborate plots, emotional investment in characters, and intense or witty repartee.

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