He is best known for his science fiction novels set in the Humanx Commonwealth, an interstellar union of races including humankind and the insectoid Thranx. Many of these revolve around the character of Philip Lynx ("Flinx"), an empathic young man who must save the universe. Flinx's constant companion since childhood is a minidrag named Pip, a flying, empathic snake capable of spitting a highly corrosive and violently neurotoxicvenom.
In the area of fantasy, his best-known work is the Spellsinger series, in which a young musician is summoned into a world populated by talking creatures where his music allows him to do real magic whose effects depends on the lyrics of the popular songs he sings (although with somewhat unpredictable results).
Many of Foster's works have a strong ecological element to them, often with an environmental twist. Often the villains in his stories experience their downfall because of a lack of respect for other alien races or seemingly innocuous bits of their surroundings. This can strongly be seen in such works as Midworld with a semi-sentient planet that is essentially one large rainforest and Cachalot an ocean world populated by sentient cetaceans. Foster usually devotes a large part of his novels on descriptions of strange environments of alien worlds and how the flora and fauna of these worlds exists together. Perhaps the most extreme example of this is Sentenced to Prism where the protagonist finds himself trapped on a silicon-based world rather than the more common carbon-based.
Foster has been so prolific that he is often rumored to have been the ghostwriter on novels with which he had little direct involvement, such as the novelization of The Motion Picture which was credited to (and actually written by) Gene Roddenberry. However, it has recently become known that he did co-write the original novelization of Episode IV A New Hope, which had been credited solely to George Lucas, and was responsible for the original story treatment for Star Trek: The Motion Picture.