In-betweening, also called tweening, is the animation process of creating intermediate frames between two keyframes to produce smooth motion. In traditional hand-drawn animation, lead animators draw the key poses while assistant animators fill in the in-between drawings that create fluid transitions between those poses.

The quality and timing of in-betweens profoundly affects how animation feels. Spacing the in-betweens closer together creates slow, smooth movement, while wider spacing produces faster, snappier action. The arc and path that in-betweens follow determines whether motion feels natural and organic or stiff and mechanical. In digital animation, in-betweening can be handled through interpolation algorithms, but careful manual adjustment is often required to achieve natural, appealing motion that respects the principles of animation.

In AI video generation, in-betweening logic is handled automatically by the model as it generates frames between keyframes or extends video from starting frames. Understanding in-betweening as a concept helps creators evaluate whether AI-generated motion feels natural and can inform decisions about frame rate, motion speed, and when to provide additional keyframes to guide the generation process.