The Hajj (حج‎) is the annual pilgrimage to Makkah, Saudi Arabia. It is currently the largest annual pilgrimage in the world, and is the fifth pillar of Islam, a religious duty that must be carried out at least once in their lifetime by every able-bodied Muslim who can afford to do so. The Hajj is a demonstration of the solidarity of the Muslim people, and their submission to GOD (Allah in the Arabic language). The pilgrimage occurs from the 7th to 13th day of Dhu-al-Hijjah, the 12th and last month of the Islamic calendar. Because the Islamic calendar is a lunar calendar, eleven days shorter than the Gregorian calendar used in the Western world, the Gregorian date of the Hajj changes from year to year. In 2007, the Hajj was from December 17–21; in 2008 from December 6–10, and in 2009 it was from November 25–29. Ehram is the name given to the special state in which Muslims live whilst on the pilgrimage.

The Hajj is associated with the life of Islamic Last Holy Prophet Muhammad صلی علیہ و آلہ و سلم from the 7th century, but the ritual of pilgrimage to Makkah is considered by Muslims to stretch back thousands of years to the time of Prophet Ibrahim. Pilgrims join processions of hundreds of thousands of people, who simultaneously converge on Mecca for the week of the Hajj, and perform a series of rituals: Each person walks counter-clockwise seven times about the Kabah, the cube-shaped building which acts as the Muslim direction of prayer, kisses the Black Stone (Hajr-e-Aswad) in the corner of the Kabah, runs back and forth between the hills of Al-Safa and Al-Marwah, drinks from the Zam Zam Well, goes to the plains of Mount Arafat to stand in vigil, and throws stones in a ritual Stoning of the Devil (Rammi). The pilgrims then shave their heads, perform a ritual of animal sacrifice (Qurbani), and celebrate the global festival of Eid-ul-Adha.

As of 2009, more than three million pilgrims participate in this annual pilgrimage. Crowd-control techniques have become critical, and because of the large numbers of people, many of the rituals have become more stylized. It is not necessary to kiss the Black Stone, but merely to point at it on each circuit around the Kabah. Throwing pebbles was done at large pillars, which for safety reasons in 2004 were changed to long walls with catch basins below to catch the stones. The slaughter of an animal can be done either personally, or by appointing someone else to do it, and so forth. But even with the crowd control techniques, there are still many incidents during the Hajj, as pilgrims are trampled in a crush, or ramps collapse under the weight of the many visitors, causing hundreds of deaths. The Kingdom of Saudi Arabia's Ministry of Hajj has a website, with the message, "Be peaceful, orderly and kind. No crushing."

Pilgrims can also go to Makkah to perform the rituals at other times of the year. This is sometimes called Umrah. However, even if they perform the Umrah, they are still obligated to perform the Hajj at some other point in their lifetimes if they have the means to do so.