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.
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.
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. MGM had this scheduled for US filming in Spring 1958 and Spring 1959, and postponed both times. Colman 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. 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.