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.
The jarring theme music by Dominic Frontiere was originally heard in "The Outer Limits: The Form of Things Unknown" in 1964.
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. --INCORRECT: it is actually the FIFTH finger that sticks out. This is shown repeatedly, in close-ups. The pinky is the fifth finger, the thumb being the first. The 4th doesn't stick out at all, it's consistent with the first four fingers.
The effect that was used for the scenes showing the aliens vanishing after glowing red, wasn't seen until the third episode.