Around 2.5 million Muslims from around the world have begun their holy journey to the city of Mecca in Saudi Arabia.
Muslims from more than 160 countries including the UK begin their journey by gathering together for prayers in one of the big mosques.
There's lots of security to make sure everything goes smoothly, but it's been raining heavily this year so hundreds of thousands of umbrellas are being used.
There are worries that swine flu could spread because there are so many people very close together, which is why some wear face masks.
Every Muslim is expected to go to Mecca to take part in the Hajj at least once in their life, as long as they are healthy and can afford to get there. Even children go.
On their pilgrimage Muslims spend a lot of time praying and reading their holy book the Koran.