The show was originally to have been called "Solo", but in the year it was due to come out the movie Goldfinger (1964) was released with a villain called "Solo."
The stunts were usually done by actors David McCallum and Robert Vaughn as well as by stuntmen, and the best version was used. McCallum wasn't too keen on heights and Vaughan wasn't too keen on water - so they tried to avoid those stunts.
U.N.C.L.E. stands for "United Network Command for Law and Enforcement"
Access to U.N.C.L.E. headquarters was made through Del Floria's, a tailor's shop. Additional entrances were mentioned but not seen.
In the original, unaired version of the pilot episode (shot in color), the Head of U.N.C.L.E. was called Mr. Allison, played by Will Kuluva. When the program finally aired it was shown entirely in black and white with Kulava's scenes re-shot, featuring Leo G. Carroll as Mr. Waverly.
At one point, the producers considered combining the show with "The Girl from U.N.C.L.E." (1966) to make a single, two-hour series called "The U.N.C.L.E. Show".
Ian Fleming was reportedly one of the consultants when this series was being planned. The name Solo was borrowed, apparently with his blessing, from the novel Goldfinger.
Napoleon Solo was originally to have been a Canadian. Although Ian Fleming assisted in the creation of the series, at one point EON Productions - which owned the rights to Fleming's novel "Goldfinger" - threatened legal action over the use of the name Napoleon Solo.
The show's third season saw a change of style that resulted in the amount of comedy being increased in response to the "camp" craze made popular by _"Batman" (1966/II)_ and "Get Smart" (1965). As a result, UNCLE's ratings plummeted and the series never recovered. Despite a return to serious stories the next year, it was cancelled midway through its fourth season.
The meaning of the acronym THRUSH was never spelled out in the series, though a meaning was created for one of the UNCLE novels published at the time ("Technological Hierarchy for the Removal of Undesirables and the Subjugation of Humanity").
Except for "Alexander the Greater Affair, episodes were always titled "The (insert episode title) Affair", while each act also carried its own title, usually taken from a line of dialog.
Robert Vaughn worked on his PhD during the course of the series, and often was allowed to leave the set early so that he could attend night classes.
When they were filming a reunion movie, both lead actors were asked how the success of this show affected their careers. David McCallum said that he was often typecast and found it difficult to play other types of roles, Robert Vaughn said in his case the opposite was true, he played nothing but villains after the series ended.
The name Illya Kuryakin comes from a Broadway play written and directed by Jules Dassin. "Ilya Darling" is Dassin's musical adaptation of his earlier film "Never on Sunday," or in Greek "Pote tin Kyriaki". The irony lies in the fact that the Kuryakin character is Russian and Dassin was blacklisted for years as a suspected Communist.
Originally, Will Kuluva was to play the head of U.N.C.L.E but was replaced by Leo G. Carroll. After the pilot was screened for the network executives, they told the producers to "get rid of the foreign guy". They really meant for David McCallum's Russian agent to get the ax, but the producers thought they meant Kuluva.