The eerie effect of the children's glowing eyes was created by matting a negative (reversed) image of their eyes over the pupils when they used their powers. The British print of the film contained no optical effects as the British Board of Film Classification considered them too frighting for an 'A' classification.

Based on the John Wyndham novel, "The Midwich Cuckoos". The title refers to the fact that when cuckoo birds lay eggs, they deposit them in the nests of other (unsuspecting) birds, who then raise the cuckoo chicks as their own. Compounding the insidious nature of this process, the cuckoo chicks often kill their nestmates in competition for food and parental attention.

Originally begun in 1957 as an American picture, writer Stirling Silliphant said he wrote it with Ronald Colman in mind, but at the time producer Milo O. Frank Jr. was saying he wanted Glenn Ford to star. M-G-M had this scheduled US filming for Spring 1958 and Spring 1959 and postponed both times. Coleman was gravely ill and housebound, dying soon after the script was completed, in an odd twist, his replacement was George Sanders, who had recently married Benita Hume, Colman's widow. Glenn Ford was one of MGM's few contracted big-named stars at the time and was in high demand for other MGM projects.

The map co-ordinates for Midwich, given over the radio when pilots are being advised to avoid the local airspace, refer to the real life village of Woodmancott in Hampshire.

After little David opens the Chinese box for the chocolate, his father takes the box to another house where a blonde girl to also open the box. In the meantime, her mother is standing off to the side with another blonde boy in her arms. The boy child and the girl are played by the same actor, Kim Clarke Champniss. The little 'girl' is wearing a very obvious wig, and their faces are identical.

When the priest is asked about the pregnant women, he mentions that one of them is a girl by the name Evelyn Harrington. George Sanders had previously starred in and won an Oscar for the movie All About Eve (1950), and the full name of that movie's title character was Eve Harrington.

When the military are testing Midwich for radiation after everyone wakes up, there is a sign in the background that says "Beware of Children". Another "Beware of Children" sign appears towards the start of the car crash scene.

Although the original British release of the film was premiered several months prior to the US release there is nothing to indicate that both versions of the film were not edited at the same time.

Just like the film, there are British and American versions of the novel that are subtly different.

The blonde wigs that the children wore were padded to give the impression that they had abnormally large heads.

Martin Stephens had already worked with George Sanders in A Touch of Larceny (1960), playing his nephew.

The plane crash footage at 12 minutes is used again in Echo Of Diana (1963)

Italian censorship visa # 33420 delivered on 23-11-1960.

Clive Powell is the only actor who appears in this film and its sequel Children of the Damned (1964). In both films, he portrays the leader of the alien children (albeit only briefly in the first one).

Child star Martin Stephens dies in his two most famous movies: this one and The Innocents (1961).