.hack is a series of single-player action role-playing video games developed for the PlayStation 2 console by CyberConnect2 and published by Bandai. The four games, .hack//Infection, .hack//Mutation, .hack//Outbreak, and .hack//Quarantine, all feature a "game within a game", a fictional massively multiplayer online role-playing game (MMORPG) called The World which does not require the player to connect to the Internet. Players may transfer their characters and data between games in the series. Each game comes with an extra DVD containing an episode of .hack//Liminality, the accompanying original video animation (OVA) series which details fictional events that occur concurrently with the games.

.hack//Sign is a Japanese anime television series directed by Kōichi Mashimo, and produced by studio Bee Train and Bandai Visual, that makes up one of the four original storylines for the .hack franchise. Twenty-six original episodes aired in 2002 on television and three additional bonus ones were released on DVD as original video animation. The series features each characters designed by Yoshiyuki Sadamoto, and written by Kazunori Itō. The score was composed by Yuki Kajiura, marking her second collaboration with Mashimo.