Around two million Muslims have travelled to Mecca in Saudi Arabia for a pilgrimage which is an important part of their religion.
The pilgrimage is called Hajj. People who follow the Islamic faith are meant to take part in it at least once during their lifetime if they can afford it.
They are in Mecca for prayers and special ceremonies. The events start on Thursday and continue until Monday.
Many pilgrims are elderly and have been saving all their lives to take part.
Last weekend, more than 1.5m pilgrims had arrived in Mecca, with more getting there every day.
In previous years, there have been tragedies at the Hajj, when pilgrims have been killed in crushes caused by so many people being in the same place.
But this year the authorities have taken steps to make sure there is enough room for everyone, and have set up lots of medical teams to help anyone who gets ill.