The former Cardinal Joseph Ratzinger had an intimate view as Pope John Paul II, with whom he had worked closely for nearly a quarter-century, suffered through the debilitating end of his papacy.
The announcement took the Vatican - and the rest of the world - by surprise.
Several cardinals on Monday didn't even understand what Benedict had said during the consistory, said the Rev. Federico Lombardi, the Vatican spokesman said. Others who did were stunned.
"All the cardinals remained shocked and were looking at each other," said Monsignor Oscar Sanchez of Mexico who was in the room when Benedict made his announcement.
Benedict was born April 16, 1927 in Marktl Am Inn, in Bavaria, but his father, a policeman, moved frequently and the family left when he was 2.
In his memoirs, Benedict dealt what could have been a source of controversy had it been kept secret - that he was enlisted in the Nazi youth movement against his will when he was 14 in 1941, when membership was compulsory. He said he was soon let out because of his studies for the priesthood. Two years later he was drafted into a Nazi anti-aircraft unit as a helper. He deserted the German army in April 1945, the waning days of the war.
He called it prophetic that a German followed a Polish pope - with both men coming from such different sides of World War II.
Benedict was ordained, along with his brother, in 1951. After spending several years teaching theology in Germany, he was appointed bishop of Munich in 1977 and elevated to cardinal three months later by Pope Paul VI.
John Paul named him leader of the Congregation for the Doctrine of the Faith in 1981 and he took up his post a year later. Following John Paul's death in 2005, he was elected pope April 19 in one of the fastest conclaves in history, just about 24 hours after the voting began.