The Cthulhu Mythos is the system of imaginary entities, books, and locations initially invented by Lovecraft and shared with other writers. Carter takes particular interest in noting the stories where particular aspects of Mythos lore first appeared, and tracing their reappearances in later tales.
The book takes pains to establish whether each Lovecraft story "belongs to the Cthulhu Mythos" or not. His requirement for including a story on the list of Mythos stories is that it must "present us with a significant item of information about the background lore of the Mythos, thus contributing important information to a common body of lore."