Emil Chronicle Online, often referred to as ECO, is a massively multiplayer online role-playing game created by Headlock and published by GungHo. It was first released in Japan on November 24, 2005 for Microsoft Windows. Gravity purchased the worldwide distribution rights for all regions except Japan from GungHo Online Entertainment Inc. in November 2005.

On December 18, 2006, Infocomm Asia Holdings licensed the game for release in Southeast Asia and Oceania. Currently, there is no set release date for this version. On January 17, 2007, The9 licensed the game for Mainland China. On May 9, 2007, Gravity partnered with NHN Corp. to bring "Gravity Zone", a portal website which will eventually "offer up to 12 online games and "Emil Chronicle Online" will be the first online game offered through "Gravity Zone" commencing from the end of May 2007".

Emil Chronicle Online takes place in the land of Acronia. There exists three races; Emil (human race), Titania (angels), and Dominion (demons). In relation to our understanding of the worlds Tatania is above the world of Acronia, while Dominion is below it. In the distant future the civilization completely depleted the earth’s valuable resources. In its search for new resources worlds which were never meant to be connected were bridged. Complete chaos ensued between the three races. Humans entered into the heavens and underworlds, realms never meant for them. From the aftermath of chaos was the emergence of a new history. There was a great effort to rebuild and regroup as each race reclaimed their rightful spot in the world.

Within the game itself is a virtual theater operated by a well-known anime production company of the same name. The area, which looks like a movie theater will show promotional clips of exclusive content and staff interviews. In addition, players can purchase Production I.G merchandise and other in-game items.

Currently the commercial version of the game costs 1,500 yen (12.40 USD) for 30 days. The game does offer a free trial period.