Tweening, short for in-betweening, is the process of generating intermediate frames between two images to give the appearance that the first image evolves smoothly into the second image. Inbetweens are the drawings between the keyframes which help to create the illusion of motion. Tweening is a key process in all types of animation, including computer animation. Sophisticated animation software enables one to identify specific objects in an image and define how they should move and change during the tweening process.

Traditional inbetweening involves the use of drawing tables with underneath lighting to draw a set of pencil-on-paper pictures, but in more recent times computers may be used to speed up the inbetweening process. The use of computers for inbetweening was pioneered by Nestor Burtnyk and Marceli Wein at the National Research Council of Canada. They received a Technical Achievement Academy Award in 1997, for "pioneering work in the development of software techniques for computer assisted key framing for character animation".

Contrary to popular belief no animator really draws inbetweens for all 24 frames required for one second of film. Only very fast movements require animation "on ones" as it is called. Most movements can be done with 12 drawings per second, which is called animating "on twos". Too few inbetweens distort the illusion of movement, like in cheap TV animation series where there can be as few as 4 drawings for a second of film.