Xenosaga (ゼノサーガ Zenosāga) is primarily a series of video games developed by Monolith Soft and published by Namco. Xenosaga's main story is in the form of a trilogy of PlayStation 2 video games. Several side stories and spinoff projects have also been developed, as well an anime adaptation.
The Xenosaga series serves as a spiritual successor to the game Xenogears, which was released in 1998 for the PlayStation by Squaresoft (now Square Enix). Outside of allusions and homages, there is no relationship between the two storylines, although it is was once commonly thought that Xenogears would somehow fit into the Xenosaga storyline. The creator of both Xenogears and Xenosaga is Tetsuya Takahashi, who left Squaresoft in 1999 along with Hirohide Sugiura. Using funds from Namco, they restarted Monolith Soft and the Xenosaga project.
The first game in the primary trilogy, Xenosaga Episode I: Der Wille zur Macht, was released in February 2002 in the Japanese market, and then in February 2003 in the American market. Xenosaga Episode II: Jenseits von Gut und Böse was released in June 2004 in Japan and February 2005 in North America. Xenosaga: The Animation, an anime based on Episode I, premiered on TV Asahi in Japan on January 5, 2005. Xenosaga: Pied Piper, a 3 chapter-long cellphone-based game depicting the history of cyborg "Ziggurat 8" 100 years before the start of Episode I, was released in Japan in July 2004. Pied Piper was not released in the United States. Xenosaga Episode III: Also sprach Zarathustra was the final title in the Xenosaga series, effectively cutting the initial projection of the series in half. A retelling of the first two episodes (titled Xenosaga I & II) for the Nintendo DS has been announced. This release is rumored to be the final version of Xenosaga DS. 
All three episodes of the main Xenosaga trilogy are named after the books of Friedrich Wilhelm Nietzsche. Several Nietzschean concepts and references appear throughout the series.