‘Yog-Sothoth’ is a basically immature conception, & unfitted for really serious literature...

H.P. Lovecraft to Frank Belknap Long, 22 February 1931

The “Cthulhu Mythos” is a name given to the superficial elements of Lovecraft’s tales: the fictional New England towns; the extraterrestrial “gods”, and the magical grimoires (see “His Creations”). However, Lovecraft never used the term “Cthulhu Mythos” himself, on rare occasions referring to his series of connected stories as his “Arkham cycle.” Instead, the term “Cthulhu Mythos” was coined by August Derleth after Lovecraft’s death. As such, one could easily make the argument that Lovecraft never wrote any Mythos stories.
These elements have been used by a multitude of writers, several of them members of the “Lovecraft Circle.” The Mythos has so captured the imaginations of readers that it is perhaps better known (and more widely read) than Lovecraft’s own work. Indeed, many items of popular culture that claim to be inspired by Lovecraft are, in reality, more inspired by the Mythos. However, the works of H.P. Lovecraft and those of the Mythos should be considered as different phenomena.
Many horror authors began their careers writing Mythos fiction, eventually moving on and finding their own voices. Some of the members of the Lovecraft Circle that incorporated elements of the Mythos into their own work included Robert Bloch, August Derleth, Robert E. Howard, Henry Kuttner, Frank Belknap Long, and Clark Ashton Smith. Later authors who also continued this tradition include Ramsey Campbell, Lin Carter, Stephen King, T.E.D. Klein, Brian Lumley, and Colin Wilson.
S.T. Joshi’s The Rise and Fall of the Cthulhu Mythos (purchase in hardcover from Amazon.com or Barnes & Noble) provides a complete history of the Mythos, from its genesis in Lovecraft’s works up to the present. An excellent bibliography of Mythos fiction is Chris Jarocha-Ernst’s A Cthulhu Mythos Bibliography & Concordance (purchase in paperback from Amazon.com or Barnes & Noble), which lists an astounding 2,631 stories. Daniel Harms’ The Cthulhu Mythos Encyclopedia (purchase in paperback from Amazon.com or Barnes & Noble) and The Necronomicon Files (purchase in hardcover from Amazon.com or Barnes & Noble or in paperback from Amazon.com or Barnes & Noble) are also excellent resources about the Mythos.
Since The H.P. Lovecraft Archive is devoted to the study of H.P. Lovecraft and not that of the Mythos, only a limited amount of information on the latter is found here. If you’d like more information on the Cthulhu Mythos, we recommend the following sites:

The Cthulhu Mythos: A Guide (Joseph Morales)

A very thorough site which features the Cthulhu Universalis (an encyclopedia and concordance of the Mythos), The Cthulhu Mythos: An Annotated Bibliography, and A Short Tour of Lovecraftian New England.

Innsmouth Free Press

A Canadian “micro-publisher” providing daily information on Lovecraft and the Mythos including articles, interviews, and reviews, as well as an electronic magazine of short fiction.

Letters from Outside (Richard D. Magrath, Duane Pesice, and J.G.W. Russell)

This site features a large collection of original Mythos fiction, articles, humor, and poetry.

MythosWeb (Peter F. Guenther)

An annotated index to published collections of Cthulhu Mythos fiction, organized by author, year, and rating.

The NetherReal (Jim Hawley)

This site, devoted to the fiction and art of the Mythos, also includes The Cthulhu Lexicon (a guide to characters, creatures, items, and locations), the Codices of the Mythos (a list of books and their contents), and When The Stars Are Right... (a timeline of events).

Reader’s Guide to the Cthulhu Mythos (Edward P. Berglund)

The centerpiece of this site is The Cthulhu Mythos on the Internet, a list of over a thousand Mythos stories, articles, and reviews that can be read online. It is also home to Nightscapes, an electronic magazine featuring Mythos fiction, articles, art, poetry, and reviews.

The Temple of Dagon

This site includes Mythos news items, reviews, fiction, artwork, and articles. It’s also now the home to Daniel Harms’ The Official Cthulhu Mythos FAQ.