Star Trek was nominated for an Emmy Award for the special effects in this episode.
This was the only appearance of the Tholians in the "Star Trek" franchise until Future Tense (2003) 35 years later.
This is one of the few episodes in which all of the regular second and third-season characters, Kirk, Spock, McCoy, Scott, Sulu, Uhura, Chekov, and Chapel, appear.
This episode is the only time that Spock refers to McCoy by his nickname, Bones.
According to James Doohan, NBC executives told him to comb his hair back for the third season. Doohan hated wearing his hair this way and stopped doing so during the filming of 'The Tholian Web'.