Alternate timelines are settings where an alternate history is occurring. These generally exist in parallel with a main timeline and may be either stable or unstable and destined for collapse. Alternate timelines may be created by time travel, science experiment gone wrong, or may always exist. They may be reached by portal, wormhole, time machine, superpower, or other means. They may be another dimension. They are home to time travelers, Nazis, and unimaginable horrors.

Worldbuilders creating alternate timelines may wish to design an entire alternate history for these locations. Creating a timeline may also be useful. Like alternate histories, alternate timelines may diverge from our own timeline at a single event.

Alternate timelines offer a narrative opportunity to examine choices, the consequence of those choices, and the length a person may go to change the past.