Visions of the Virgin - part of the Roman Catholic tradition since the Middle Ages - have been reported at numerous venues around Europe and Latin America. There have been countless sightings in Ireland, where the Church is still part of the fabric of life.
Lourdes, an isolated rural French hamlet where a peasant girl called Bernadette claimed that she saw the Virgin Mary some 140 years ago, is probably the most important place of pilgrimage for devout Catholics. Five million believers, many of them sick or disabled, visit the shrine at Lourdes every year, hoping for a cure.
Another principal site is Fatima in Portugal, where the Virgin is said to have appeared to three shepherd children in 1917. Lourdes and Fatima are recognised by the Vatican, as is Knock in western Ireland. But other centres of pilgrimage have yet to be given the official seal of approval.
Only last month, the Vatican ruled that there was no evidence of "supernatural visions" at the shrine to the Virgin at Medjugorje in Bosnia, a venue venerated by many devout Catholics. Sightings have been reported there since six children claimed they saw the figure of Mary appear and speak to them in 1981. Local Franciscan friars declared the vision authentic.
The Vatican's ruling was made as the Pope prepared to visit a shrine at Marija Bistrica, near Zagreb, in neighbouring Croatia, which has been dedicated to the Virgin since the 15th century.
In Achill Sound, a village in Co Mayo, busloads of pilgrims flock to the house of Christina Gallagher, who claims to receive messages from the Virgin and to suffer the stigmata.