Regulated by ancient traditions and recent rules, the period between popes -- known by the Latin term "interregnum" -- will begin exactly at 8 p.m. Rome time Feb. 28.
Some leaders know which moments or decisions in their lives will lead all biographies of their lives. Winston Churchill knew that his leadership during World War II would be the central theme of all future historians looking at his life. John F. Kennedy understood that his handling of the Cuban Missile Crisis would define his role in the history books: If he messed that up, nothing else would matter.
Opinion: Pope Benedict should bar Los Angeles Cardinal Roger Mahony from the gathering that will elect the next pope because of his role in the sex abuse scandal.
Photos: Pope Benedict XVI was elected pope in April 2005. Here's a look back at the last eight years of his papacy.
U.S. Secretary of State John Kerry says the U.S. government is "grateful to His Holiness Pope Benedict XVI for his leadership of and ministry to the world’s 1.2 billion Catholics."
"He has been a man of action and principle, working to promote human rights and dignity in places around the globe where they are too often denied, and a voice of clarity and conviction about our obligations as stewards of a fragile planet," Kerry, himself a Catholic, said in a statement.
FutureChurch, a church reform group based in Ohio, congratulated Pope Benedict XVI in a statement Monday "for his wise decision to resign," saying "it takes courage to recognize when one no longer has the physical strength and health to fulfill the responsibilities of one of the world's most demanding jobs."
While the main group which advocates for the ordination of women to the Roman Catholic priesthood said it is "saddened" by news of Pope Benedict's deteriorating health, it also said the pope "used his power to take significant steps backwards for women."
Cardinal Roger Mahony, the retired archbishop of Los Angeles who was been publicly disgraced in past weeks over his handling of priests accused of sex abuse in the 1980s, has confirmed he will be voting to choose the next pope.
Writing in a statement posted to the Los Angeles archdiocese's website, Mahony recalls that he took place in the 2005 conclave that selected Pope Benedict.
New York Cardinal Timothy Dolan, the president of the U.S. bishops' conference says he was "as startled as the rest of you" to learn of Pope Benedict's resignation.
Appearing on NBC's Today Show this morning, Dolan, who was appointed a cardinal by Pope Benedict last year, said the news made feel "somber."
"I find myself kind of somber, in a way," Dolan said on the show. "Boy, I love this pope. Every Catholic feels the pope is his father, we call him our Holy Father. The world looks to him with respect and affection."