Many of Farmer's works involve reworking existing characters from fiction and history, such as Around the World in Eighty Days. His favorite subjects for this type of work are the pulp heroes Tarzan and
Doc Savage.
This has led to a burgeoning of a particular type of this form of fiction which is frequently referred to by reference to Farmer's original premise, the Wold Newton family.
His
Riverworld series follows the adventures of Richard Burton and Samuel Clemens through a bizarre afterlife in which every human to ever have lived is simultaneously resurrected.
Farmer wrote
Venus On the Half-Shell under the name Kilgore Trout, a fictitious author who appears in the works of Kurt Vonnegut.
Farmer's works often contain sexual themes: his collection of short stories
Strange Relations was a notable event in the history of sex in science fiction.
His parallel universe series World of Tiers inspired Roger Zelazny's Amber series.
His work sometimes contains religious themes and has included Jesus Christ as a character in both the Riverworld series and
Jesus On Mars. (To be specific, Jesus shows up in the Riverworld setting during a short story in
Riverworld and Other Stories and is not a character in the primary books of the Riverworld series.)