The setting first appeared in a 1992 RPG scenario[1][better source needed] and revolves around a secretive organization tasked with protecting the United States from paranormal and alien threats.

Arc Dream Publishing also made a partnership with Pelgrane Press to release a prequel named The Fall of DELTA GREEN using the Gumshoe System in 2018.

Delta Green agents work undercover through other U.S. government agencies, recruiting across a wide range including the FBI, ATF, CIA, CDC, DEA and the U.S. Military.

Delta Green must not only contain incursions from creatures from the Cthulhu Mythos, but also secret conspiracies and rogue organizations that seek to weaponize its powers.

Majestic was weakened and destroyed, while Delta Green has been brought back into the government fold, given a budget and official standing, using the War on Terror as a cover to their operation.

The group was introduced in the seventh issue of The Unspeakable Oath, a Call of Cthulhu fanzine created by Pagan Publishing, in early 1993.

The Delta Green supplement lays the groundwork for organized investigations into paranormal crime and horror, setting up the initial plot and providing players with their motivations and the resources they need to carry out their tasks.

Delta Green: Countdown introduced counterparts in the United Kingdom and Russia, called PISCES and GRU SV-8 respectively, as well as real-life international law enforcement and intelligence agencies.

The 2016 standalone edition takes the percentile dice of Basic Role-Playing and Call of Cthulhu mechanics, and introduces modifications adapted for the setting.

[6] Fall of Delta Green also introduces new mechanics for the Gumshoe formula, adapting the new rules introduced by the standalone edition of Delta Green: Bonds and Motivations complement the sanity system (Itself based on the one used in Trail of Cthulhu), adaptation to SAN loss was included, Revelation works similarly to the "traumatic background" in Delta Green's character creation and a few new abilities were added, most prominently was the ability of "Agency" to represent the capability of the Player Character to manipulate their employing agency's (Such as the CIA, MACV-SOG or the Federal Bureau of Narcotics) resources in their investigations.

[7] The setting was first introduced in 1992 in the seventh issue of The Unspeakable Oath, Pagan Publishing's Call of Cthulhu fanzine, in the scenario Convergence, which would be later republished in the first Delta Green sourcebook.

The scenario introduced Delta Green as a loose organization within the U.S. government dedicated to secretly fight the creatures from the Cthulhu Mythos.

[9] On May 1, 2006, the Pagan Publishing homepage was updated with an announcement describing the reasons for the delay of the new release of Delta Green, and stating that the d20 edition of the game should be available in time for Gen Con Indy.

[14] Arc Dream Publishing released an ebook edition of the novel Delta Green: Denied to the Enemy in January 2011.

[15] As of October 29, 2015, a Kickstarter Campaign by Arc Dream Publishing funded a series of new Delta Green products, converting the setting into a standalone role-playing game.

On April 18, 2019, the sourcebook Delta Green: The Complex was released, containing information on federal agencies and related professions that didn't appear in the Agent's Handbook, such as the ATF, NSA, Booz Allen Hamilton, United States Coast Guard among other federal law enforcement, intelligence, military and private contractors agencies that work for the US government.

It is Delta Green's first full-length campaign, centered around Hastur, The King in Yellow and Carcosa, with a bigger focus in surreal horror.

[30] Scenarios by Dennis Detwiller, published online and (since Sep 2014) through Patreon: Adam Scott Glancy wrote a regular column titled "Directives from A-Cell" in the first six issues of Worlds of Cthulhu magazine.

He concluded by giving the book at excellent rating of 9 out of 10, saying, "One of the biggest, most detailed, best conceived and inspiring sourcebooks ever released for any game system.

"[31] In the July 1998 edition of Dragon (Issue 249), Ray Winninger called Delta Green "one of the richest, most detailed, and most complete sourcebooks ever published."

Winninger found that this book "successfully melds Lovecraft's eerie mythos with modern UFO and conspiracy folklore to produce a compelling hybrid setting all its own."

"[32] In Issue 3 of the French game magazine Backstab, Stéphane Bura wrote, "Seldom has a role-playing product inspired in me such a sense of despair and helplessness in the face of the threat it offered."

It doesn't always ring true, but it represents a vast improvement over sanity systems that feature pulp-inspired caricatures of mental health.