thumb|400px|right|Gregorian - Ordinary worldGregorian is a German band, headed by Frank Peterson, which performs Gregorian chant-inspired versions of modern pop and rock songs. Because it features both vocal harmony and instrumental accompaniment, the music cannot be considered true Gregorian chant.
Originally, Gregorian was conceived as a more pop-oriented group in the vein of Enigma. Under this concept, they recorded the 1991 album Sadisfaction, with lead vocals provided by The Sisters of Oz: Susana Espelleta (Peterson's wife at the time) and Birgit Freud. However, this was the only album in that style.
In 1998, Peterson and his team re-invented the project to perform popular songs in the Gregorian style. The criteria for song selection were strict; in order to be considered, a song needed to be translatable into the 7-tone scale. After the songs were chosen, twelve vocalists - previously acclaimed session and choir singers - were hired.
Each Gregorian album is initially digitally tracked at Nemo Studios, Peterson's Hamburg studio. The vocalists then record their parts in a church atmosphere with dimmed lights and candles, in order to escape what Peterson referred to in a 2001 interview as the "cold and technical" studio atmosphere.
The concept proved to be successful, and the group proceeded to record several more Masters of Chant albums in the same style. Their 2004 album, The Dark Side, was a slight departure from the others, featuring a darker repertoire consistent with the title.
In 2005, The Masterpieces, a compilation album with a live DVD, was released. A fifth Masters of Chant album was released on 31 March 2006. In 2006, a festive album was also released, titled Christmas Chants. A sixth Masters of Chant album was released on 28 September 2007. In 2009 a seventh Masters of Chant album followed.