Mueller's appointment to a powerful job the pontiff himself held for more than two decades - prefect of the Congregation for the Doctrine of the Faith - comes at a time when the Vatican is beset by controversies including allegations of corruption and internal intrigues, as well as a lingering child abuse scandal.
Mueller will have the delicate task of overseeing issues ranging from overhauling Church practice in the wake of the scandal of child sexual abuse by priests to reconciling the position of dissident ultra traditionalist movements.
The 64-year-old bishop of the southern German city of Regensburg, where the pope himself taught theology in the 1960s, is a well respected conservative theologian and the author of some 400 publications.
Unusually for a bishop elevated to high office under the conservative Benedict, he has links to the politically-tinged liberation theology movement of South America, which has been sharply criticized by both Benedict and his predecessor John Paul II.
Mueller is overseeing publication of the collected works of Joseph Ratzinger, who took the name Benedict when he became pope. But among his own publications is a work written with liberationist theologian Gustavo Gutierrez, call...