GURPS Middle Ages I is a supplement which focuses on English history going back from the Dark Ages to the Renaissance, covering topics including Saxon law as it developed, the Celtic church and its influence, and the effects of the Hundred Years' War.

The book includes a chapter on spellcasting with simple rules for rune magic as well as Hellenistic charms.

The book also includes a "Medieval Bestiary" built from background notes and game statistics for mythological creatures taken from that era.

[1] Illustrations are by Ruth Thompson, Carl Anderson, Thomas Baxa, Angela Bostick, Dan Carroll, Evan Dorkin, Rick Lowry, and Rob Prior, with a cover by Rowena.

[1] After the Secret Service raid on SJG, the company stopped publishing adventures for financial reasons, and started releasing more standalone GURPS supplements; this included a focus on historical supplements like GURPS Middle Ages I, a subgenre that the game had yet to explore.