Up to one million people are expected in Saint Peter's Square and nearby streets of Rome to witness the canonisation of John XXIII and John Paul II, two of the great popes of the 20th century.
John XXIII, who reigned from 1958 to 1963 and set up the modernising Second Vatican Council, and John Paul II, the Pole who reigned for nearly 27 years and played a leading role on the world stage, will be declared saints by Pope Francis.
Francis' own huge popularity has added extra appeal to the unprecedented ceremony to raise two former leaders of the church to sainthood. But while both were widely revered, there has also been criticism that John Paul II, who only died nine years ago, has been canonised too quickly.
Pope Francis has declared John XXIII and John Paul II saints in a ceremony made more historic by the presence of retired Pope Benedict XVI.
The men being made saints today left their mark on the Catholic Church in different ways. Here's all you need to know about the pontiffs.