Series Creator Larry Cohen said that his inspiration for this series was Invaders from Mars (1953) and Invasion of the Body Snatchers (1956), his two favorite films when he was a kid.
The true form of the Invaders was never completely revealed, except for glimpses of the creatures in two episodes. The Invaders' human forms were unstable and had to be "regenerated" periodically, or else they would die.
Some Invaders could be identified by what David Vincent calls a "deformed fourth finger". Technically, the finger is not deformed, but immobile, always sticking out. There are hints dropped from the beginning, that the aliens are correcting this problem so that not all invaders have it. Toward the end of the second year, they seem to have eliminated the defect altogether.
The character "David Vincent" was ranked #6 in TV Guide's list of the "25 Greatest Sci-Fi Legends" (August 1, 2004 issue).
The jarring theme music by Dominic Frontiere was originally heard in "The Outer Limits: The Form of Things Unknown" in 1964.
There was never a full-size U.F.O. built for the show. The saucer was a filming miniature that was optically printed into the sky whenever it was shown flying. A full size set that represented the landing legs was built for long shot of people approaching the landed saucer. For these shots, the miniature was matted over the landing legs. Characters could even be shown climbing up into the U.F.O. in tight shots of the land legs set. Apparently the Control Room within the saucer was a semi-permanent set; the other areas within the ship changed with various episodes.
The effect that was used for the scenes showing the aliens vanishing after glowing red, wasn't seen until the third episode.
As was the case with other "Quinn Martin" productions, William Woodson provided the narration for this series.
Roy Thinnes was beginning to feel exhausted by the gruelling work schedule, which is why he doesn't have as much screen time in "The Spores" as his co-stars.
Frank Black (lead singer of Pixies) was a big fan of all things science fiction, especially this show. He wrote "Bad, Wicked World" with distinct references to it.
In his homage to 1960s Hollywood, "Once Upon a Time In Hollywood" Quentin Tarantino included a scene where Sharon Tate walks by a sign advertising "The Invaders".