A rotoscope is a device that enables animators to trace live action movement, frame by frame, for use in animation. It might be called a clumsy forerunner to digital motion capture. The device was invented by Max Fleischer, who used it in his series "Out of the Inkwell" starting around 1914. Fleischer used his brother Dave Fleischer, dressed in a clown outfit, as the live-film reference for the character Koko the Clown. The rotoscope was used in a number of later Fleischer cartoons as well, most notably the Cab Calloway dance routines in three Betty Boop cartoons from the early 1930s, and the animation of Gulliver in Gulliver's Travels.

Rotoscopy is decried by some animation purists, but has often been used to good effect. When used as an animator's reference tool, it can be a valuable time-saver. Walt Disney and his animators employed it carefully and very effectively in Snow White, primarily used in the animation of Prince Charming. By contrast, Ralph Bakshi used rotoscope quite slavishly in The Lord of the Rings and Wizards - by so doing, he could produce animation without hiring animators. Bakshi's use of rotoscoping did not meet with critical acclaim.

Poor-quality rotoscoping has slight deviations from the true line that differ from frame to frame, which when animated cause the animated line to "boil". Avoiding boiling requires considerable skill in the person performing the tracing.

Rotoscoping has often been used as a tool for special effects in live action movies. By tracing an object, a silhouette (called a matte) can be created that can be used to create an empty space in a background scene. This allows the object to be placed in the scene. However, this technique has been largely superseded by bluescreen techniques.

Rotoscoping has also been used to allow a special visual effect (such as a glow, for example) to be guided by the matte or rotoscoped line. One classic use of traditional rotoscoping was in the original three Star Wars films, where it was used to create the glowing light saber effect, by creating a matte based on sticks held by the actors.

The term "rotoscoping" is now generally used for the corresponding all-digital process of tracing outlines over digital film images to produce digital mattes. This technique is still in wide use for special cases where techniques such as bluescreen will not pull an accurate enough matte. Rotoscoping in the digital domain is often aided by motion tracking and onion-skinning software. Rotoscoping is often used in the preparation of gash mattes for other matte-pulling processes.

Richard Linklater produced a digitally rotoscoped feature called Waking Life, creating a surreal image of live action footage, a technique which is now being used to produce the movie of A Scanner Darkly.