Fly by Night is a children's or young adults' fantasy novel by Frances Hardinge, published on 7 October 2005 by Macmillan in the UK and on 25 April 2006 by HarperCollins in the US.

Hardinge's comic fantasy is set in the grotesque imaginary world of The Realm, which, as she writes in an after note, bears some similarity to early 18th century England.

The novel takes place at a time when the monarchy has been overthrown and The Realm has fractured into city states such as Mandelion, where the guilds, especially the Stationers and Locksmiths, act as power brokers.

Mosca is the daughter of Quillam Mye, an exiled writer and radical political agitator, who taught her to read but died when she was eight.

In Mandelion, a hornet's nest of political intrigue, crime and corruption, they lodge in a marriage house, which arranges weddings for people who can't afford a church.

They become inconveniently implicated in the murder of Partridge, a barge captain who transported Saracen to Mandelion, while also smuggling lead for rebels to make bullets.