As well as the comics, the Foglios have also written four Girl Genius novels, all published by Night Shade Books, and two games based on the world have been made.

[‡ 4] Girl Genius is set in an alternate-universe Europe[5] with Industrial Revolution-like surroundings, airships, and mad scientists called Sparks.

[2] These Sparks, who have incredible powers of invention,[3] turned the Age of Enlightenment into a full-scale war that ravaged the continent, until Baron Wulfenbach brought an uneasy peace through force.

[‡ 5] Girl Genius tells the story of Agatha Clay, a student and apprentice at Transylvania Polygnostic University, whose experiments never work until she encounters an electromagnetic pulse and is robbed of her locket.

[‡ 7] Agatha Heterodyne learns to mix scientific genius, a streak of heroism, and an obsessive possessiveness for what she considers her own in order to claim her heritage and birthright while trying to remain safe from the many European powers trying to use her for their own ends.

[‡ 8] The main and recurring characters of Girl Genius include: The idea for the style of Girl Genius came about when Kaja Foglio went through some of Phil's loose drawings, saying in an interview: "I was going through all of Phil's old files and I was filing all of the old sketches, and I was coming across weird airships and cats in tophats with walking canes, and all of this wonderful... Victoriana sci-fi stuff... it was like 'Oh, this is everything I love!

[‡ 20] In an interview recorded in January 2008, shortly before they began releasing pages of volume 8 of Girl Genius on their web site, the Foglios stated that they expected the climax of Volume 8 to be the rough equivalent of "the end of the first season," and that it would provide a logical break in case of author catastrophe and a fresh jumping-on point for new readers.