Trope Workshop:Parallel Universe Episode

Laconic: An episode in which the characters visit or are visited by themselves from an alternate universe or timeline.

Maybe "Alternate Universe" would be a better title?

How is this different from Alternate Reality Episode?

To quote the Alternate Reality Episode page, "In an Alternate Reality Episode, the usual main characters are absent. The main cast is still here, but they're playing "themselves" in a different role." This is for episodes when the main characters interact with their alternate selves.

Television - Live-Action

 * Star Trek
 * The Star Trek the Original Series episode "Mirror, Mirror" may be the Trope Maker. Many of the other examples parody this one in some way.
 * The mirror universe also appears in more than one Star Trek Deep Space Nine episode.
 * While the mirror universe appears in Star Trek Enterprise, the lack of crossover between the universes makes it more of an Alternate Reality Episode.
 * Stargate SG-1. The episodes "There But for the Grace of God", "Point of View", and "Ripple Effect" all dealt with "alternate realities".
 * The Community episode "Remedial Chaos Theory" establishes the existence of alternate timelines, particularly "the darkest timeline". This plays a role in the finales of season 3 and 4.

Web Original

 * Homestar Runner's Strong Bad Email #150 "alternate universe" features Strong Bad visiting the alternate versions of himself which had appeared previously in the series.

Television - Western Animation

 * Futurama - The Farnsworth Parabox. The professor (and his counterpart) accidently creates a box opening to another universe.  Many other universes are visited in this episode.
 * In the episode "It's a Mad Mad Mad Parallel World" of the Hulu-exclusive series The Awesomes, the main cast travels to an alternate universe and fights their evil counterparts with the help of the good counterpart of the villain Dr. Malocchio.
 * South Park has an episode where the boys encounter a version of Cartman from a "mirror" universe. Like the mirror Spock in the original Star Trek example, he has a goatee and mustache.