Hajj is one of the pillars and foundations of Islam, the act of worship of a lifetime, the seal of all that is commanded, the perfection of Islam and the completion of religion. Such a worship deserves that much attention to be devoted to explaining it and to detailing its essential elements, its properties, its merits, and its mysteries. This little book is translated from Kitab Asrar al-Hajj, the seventh chapter of the celebrated 12th-century work, Ihya Ulum al-Din. Al-Ghazali invites readers to dwell on the subject ofthe hajj beyond its mechanics and rituals, and to pay attention to its true substance, purpose and philosophy. And concerning it the Prophet – the blessing of God be upon him – said, 'Whoever dies without, having performed the Pilgrimage let him die, if he wish, either a Jew or a Christian.' How exalted is that act of worship without which religion is lacking in perfection, and the evader of which is equal in, waywardness to Jews and Christians: [Such a worship] deserves that much attention be devoted to explaining it and to detailing its essential elements (‘arkan), its proprieties, its merits, and its mysteries.