When Peter Lorre arrived in Great Britain, his first meeting with a British director was with Alfred Hitchcock. By smiling and laughing as Hitchcock talked, the director was unaware that Lorre, a Hungarian, had a limited command of the English language. Hitchcock subsequently decided to cast Lorre in this film, and the young actor learned much of his part phonetically.

The siege at the end of the film showing the police in a gun battle is based on the real-life Sidney Street siege, which took place on 3 January 1911 in which London police and a unit of the army's Scots Guards shot it out with a gang of heavily armed anarchists who had previously been involved in the murders of three police officers. Two anarchists and a police officer were killed.

The dentist scene was originally intended to take place in a barber shop. However, Alfred Hitchcock saw I Am a Fugitive from a Chain Gang (1932), in which there is a scene exactly like it, so he changed it to a dentist's office.

Peter Lorre's first English-language film.

The film's producer, C.M. Woolf, hated the film and only allowed it to be released as the bottom half of a double bill. Nevertheless, it won rave reviews.

The crucial cantata for the Albert Hall sequence was composed specifically for the film by Arthur Benjamin, and the same piece was used again in the remake The Man Who Knew Too Much (1956). When Alfred Hitchcock remade the movie, he offered composer Bernard Herrmann the opportunity to compose a new work for the scene, but Herrmann chose not to, citing an appreciation of Benjamin's original cantata.

The script was originally written to be a Bulldog Drummond movie, titled "Bulldog Drummond's Baby", but Alfred Hitchcock wasn't able to get the rights to the "Bulldog Drummond" name.

Apart from the opening and end credits, there is only source music in this film, i.e., music that can be heard by the characters, such as dance music in Switzerland and Wapping and the Benjamin cantata (with the rest of the concert on the radio). There is no underscoring music at all.

The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.

Alfred Hitchcock reverted back to the more familiar territory of a suspense thriller after the disastrous performance of his previous film, Strauss' Great Waltz (1934).

The title of this film comes from the name of a book written by G.K. Chesterton.

The logo for Gaumont British Pictures is located on a scarf worn by Leslie Banks during the opening scene.

Alfred Hitchcock: Just after the dentist office scene on the lamp post corner, entering stage right in a black slicker coat as a bus passes. This is at about 36 minutes running time, or 33 in some editions.