The World Youth Day (WYD) events Pope Francis will attend in Brazil later this month are a tradition started by the John Paul II during his charismatic papacy.
"You are the future of the world and the hope for the Church," his Polish predecessor, who is soon to be declared a saint, told young Catholics at the dawn of his papacy in 1978.
The first official WYD was held in 1986, although it had been preceded by two previous mass gatherings of young people from around the world in Rome.
It was followed the following year by a WYD in the Argentine capital Buenos Aires, where the current pope was archbishop until his election in March.
The gatherings have since been organised in Australia, Canada, France, Germany, Italy, the Philippines, Poland, Spain and the United States.
Following John Paul II's death in 2005 there had been doubts about whether the gatherings would continue under his more academic successor Benedict XVI.
But his first one in his German homeland in 2005 proved the doubters wrong, drawing one million people. His last one in Madrid in 2011 before his shock resignation this year was attended by 1.5 million, according to the Vatican.
The Madrid event however was marred by large protests against austerity and the cost of the event. A rainstorm also interrupted the pope's speech, prompting Benedict to commend the crowd for having faith "stronger than the rain".
Attendees are typically from local parishes and Catholic youth associations, but increasing numbers of non-believers or non-practising Catholics also attend, brought by friends or responding to invitations announced on social media.
Lay religious movements that have sprung up around the world also attend, such as the Neocatechumenal Way and Communion and Liberation. They have drawn young believers looking for non-traditional ways of practising their faith.
They have proved to be a source of energy for a troubled Roman Catholic Church, which has been shaken by thousands of child abuse scandals, ever growing secularism in the West and allegations of corruption in the Vatican.
The gatherings are often held outdoors and have a celebratory atmosphere far from the more sombre surroundings of a church. They have sometimes been described as a "Catholic Woodstock" -- the famous 1969 music festival.
At some of the events there have been reports of attendees getting a little carried away in their revelry. At the gatherings in Australia (2005) and Germany (2008) some of those attending reportedly even made use of condoms handed out by anti-WYD protesters.