Writer Jerome Bixby loosely based the script on his own short story One-Way Street.

This episode was nominated for a Hugo in 1968 for Best Dramatic Presentation.

Production of this episode was temporarily shut down when Barbara Luna came down with a high fever. “The Doomsday Machine” was filmed in the interim.

Vic Perrin (Tharn) also provides the voices of Balok in "The Corbomite Maneuver", the Metron and the Gorn in "Arena" and Nomad in "The Changeling". This is his only on-screen appearance in the series.

Star Trek: Deep Space Nine revisits the mirror universe established in this episode in several episodes starting with the season 2 episode "Crossover."

Mirror Spock: (to Mr. Kyle) Carelessness with the equipment cannot be tolerated! Your agonizer, please.

Mirror Spock: I do not wish command. But if it should befall me, I suggest you remember that my operatives would avenge my death. And some of them... are Vulcans.

Mirror Sulu: (to Kirk) Mr. Spock has orders to kill you. He will succeed. Apparently, you will also have killed him, after a fierce fight. Regrettable, but it will leave me in command...

The sash Mirror Chekov is wearing appears/disappears/reappears as he gets in the turbolift.

You can DeForest Kelley’s wedding ring during the scene where he does his “I’m a doctor, not an engineer...” line.

James Doohan can be seen in the long shots on the bridge in the final scene, but disappears in the close-ups of Shatner and Kelley.

This is the only episode in which Scotty refers to Captain Kirk as "Jim."

What Changed in the Remastered Version
The Halkan planet and the Enterprise were replaced by nicely done CGI effects. The transition to the Mirror Universe, formerly signalled by cutting together scenes of the Enterprise transiting the planet and reversals of the same scenes, was replaced by a better special effect. The I.S.S. Enterprise itself is depicted as a slightly different looking ship - more closely resembling the original model used for the two pilots, looking slightly 'heavier', and with little sensors sticking out of the front of the engine nacelles. Mr. Kyle's agonizer displayed tiny electric shocks (suggestive of a taser) when applied and left a glow that persisted for several seconds after its removal. Chekov's punishment in the Agony Booth was blurred (and the sounds were changed slightly) to suggest the presence of an energy field within the booth.