There are no known copies available of the series' first year, except for “The Frighteners” and “Girl On the Trapeze”. The episode is one of two episodes not to feature Patrick Macnee as John Steed.

The first British TV programme to be sold to a US Network.

At least three principal actors in the series went on to appear in James Bond films: Honor Blackman as Pussy Galore in Goldfinger, Diana Rigg as Tracy in On Her Majesty's Secret Service and Patrick MacNee as Sir Godfrey Tibbett in A View to a Kill.

When the series began, Ian Hendry was the main star, with the idea being he would rotate between different partners (an early version of the Mission: Impossible format). The series title actually refers to Hendry's character, Dr. David Keel, and Steed, who worked together to find those responsible for Keel's wife's murder in the first episode. Early episodes focused more on Keel's character, and Steed doesn't even appear in a couple! When the first season was interrupted by a strike, Ian Hendry quit the series during the hiatus. The same format was used for Steed with a couple of leftover Dr. Keel scripts retooled for a new character named Dr. Martin King, but other scripts, originally written for a male character, were rejigged for another new addition: Cathy Gale.

Footage of Emma Peel from one of the color episodes was reused in an episode of "The New Avengers."

Emma Peel's final episode "The Forget Me Knot", had already been filmed prior to the announcement that Diana Rigg would leave the series. After her announcement, fresh footage introducing trainee spy Tara King was filmed and these scenes spliced in with the existing footage shot for "The Forget Me Knot" in order to set up Steed's replacement partner. As she had already left the series, Rigg does not appear in this new footage - King's scenes only feature Patrick Macnee as Steed - however, Rigg did agree to return to film one brief scene where it is hurriedly explained that her husband Peter, a test pilot whose plane disappeared in the Amazon, had returned from being missing in the jungle and that Emma is returning to him. It is in this tag scene that the only meeting of Emma and Tara takes place (Emma advises that Steed prefers his tea stirred anti-clockwise), and is also the only time Steed calls Mrs. Peel "Emma". The actor who plays Emma Peel's husband in the episode "Forgot-Me Knot" is in fact Patrick Macnee himself (his face is never shown).

Mrs. Peel's maiden name is Knight.

Emma Peel's name was taken from the British film industry expression "M-Appeal", or "man-appeal", which is what the show's producers were looking for in her character.

Steed's address is 3 Stable Mews.

Emma's fighting suits were named Emmapeelers.

Diana Rigg was the first person ever to do Kung Fu on the screen. In 1965, Ray Austin went to his producers and said, "Listen, I want to do this thing called Kung Fu". They said "Kung what?" and insisted that Emma, like her predecessor, stick to judo. Instead, Austin secretly taught Rigg Kung Fu.

The original macho female spy, Cathy Gale, was a composite character based on two real-life women: Life magazine's daring photographer, Margaret Bourne-Smith, and anthropologist Margaret Mead.

To maintain the pure fantasy of it, there were strict rules about what could not be shown in an episode:

1. No "uniformed policemen"

2. No "colored people"

3. No blood.

4. No dead women.

5. No blatant sex.

At the end of "Bizarre", Mother talks directly to the audience, promising that The Avengers would return.

Production of season 4 began with Elizabeth Shepherd in the role of Emma Peel. The producers soon realized the role had been miscast, and so began again with Diana Rigg in the role.

In "Whoever Shot George Oblique Stroke XR40?", Linda Thorson speaks with her normal Canadian accent.

In one episode during the fourth season, Steed receives a Christmas card from Cathy Gale and it is postmarked "Fort Knox". This is in reference to Honor Blackman's appearance as Pussy Galore in the James Bond film Goldfinger.

"A Touch of Brimstone" was banned in the US because of Diana Rigg's Queen of Sin outfit (corset, spiked dog collar and thigh high boots), which Rigg designed herself.

Some of the Steed/Gale episodes were remade with Steed/Peel: "The Charmers" was remade into "The Correct Way to Kill" and "Don't Look Behind You" was remade into "The Joker".

In "Never Never Say Die", Emma is seen at the beginning of the show watching "The Cybernauts" on TV.

A radio series was made after the show ended; it was broadcast on South African radio during the 1970s.

Continuity Error: "False Witness" contains several goofs. When Tara escapes from the fight in the milk vat, as she runs away her clothes appear to be dry. After the getaway, she stops at a phone kiosk to call Steed, and she is completely dry, despite the fact that moments before she was swimming in milk. Later, she returns to the dairy and starts smashing bottles of milk and tipping over milk cans. When Steed arrives on the scene, the floor isn't wet and there are no broken bottles visible. Finally, with Tara in the cheese maker, we can see her from the waist up, and she doesn't appear to be swimming in milk. Yet, when Steed frees her, she is embedded up to her neck in a giant block of cheese. (There's a factual error here, too: it takes months to convert milk into cheese.)

Crew or equipment visible: In "The House that Jack Built", when the camera follows Emma Peel in the background and pans by the plastic dome in the foreground, close-up images of camera and crew can be seen in the rotating mirrors inside the dome.

American broadcasts of the 1965 season were preceded by an introduction showing Emma Peel and John Steed walking across a giant chessboard as a narrator says: "Extraordinary crimes against the people and the state have to be avenged by agents extraordinary. Two such people are John Steed -- top professional, and his partner, Emma Peel -- talented amateur. Together they are -- The Avengers!"

In some parts of the world, the opening credits for the first color season begin with a brief sequence showing Steed preparing to open a bottle of champagne. Mrs. Peel shoots the cap off the bottle, and they pour a toast to each other. Only then do the opening credits actually begin.

Some episodes of the first color season begin with a title card "The Avengers in Color."

The closing credits for the first half of the final season were a parody of the gun barrel opening sequence in the James Bond films.