Sufism is the heart of Islam. Its key is an inner light and awakening, an outer courtesy and service to humanity. It relates to it much as the mystical tradition does to Christianity, but far more prominently. With authors like Rumi achieving all-time popularity, it is increasingly acknowledged as a spiritual answer to modern materialism. This introductory guide describes its origins and background, the relationship with Islam and its spread throughout the world. It covers what is means to be a Sufi and the relevance of Sufism in modern life. Biographies of prominent Sufis and a selection of writings complete this guide to a profound tradition.