The 85-year-old pontiff, the first to resign in nearly 600 years, says his advanced age and diminishing strength have made it too difficult to carry on.
A new pope will be elected before the end of March -- right before Easter.
Statement by the Diocese of Phoenix on the retirement of Pope Benedict XVI:
"We are grateful to God for the life and leadership of Pope Benedict XVI. He has been a remarkable witness to the Gospel throughout his lifetime of service to the Church. As the successor of St. Peter, he shepherds us with wisdom, grace and love. We encourage the faithful to join us in offering up our prayers for the Holy Father as he enters into the next stage of his life."
Statement by the President on His Holiness Pope Benedict XVI:
"On behalf of Americans everywhere, Michelle and I wish to extend our appreciation and prayers to His Holiness Pope Benedict XVI. Michelle and I warmly remember our meeting with the Holy Father in 2009, and I have appreciated our work together over these last four years. The Church plays a critical role in the United States and the world, and I wish the best to those who will soon gather to choose His Holiness Pope Benedict XVI's successor."
Video: FOX 10's Diane Ryan talks to valley Catholics about the shocking news from the Vatican.