Two central Sufi concepts are tawakkul, the total reliance on God, and dhikr, the perpetual remembrance of God. Al-Muhasibi (d. 857) and his disciple Junayd (d. 910) are representative early figures. The introduction of gnostic elements (marifa) into Sufism is often attributed to Dhu-n-Nun al-Misri (d. 859). Sufism nonetheless faced growing opposition from orthodox clerics. The scholastic and ecstatic paths further diverged with the concept of fana, the dissolution into the divine, advocated by al-Bistami (d. 874), and used by Hallaj in the declaration of his unity with God, which eventually led to his execution in 922. Islamic orthodoxy and Sufism were not irreconcilable, as attested by the attempt by al-Ghazali (d. 1111) to infuse conformist Muslim religious life with mysticism.

The evolution of Sufism in the post-Ghazali period was influenced by Ibn al-Arabi and Ibn al-Farid. Their theoretical contributions led to the development within Sufism of a complex system of initiation and progression toward the Divine and set the stage for the emergence of organized Sufi orders. This phase of literary Sufism was also characterized by the prominence of Persian works, notably those of Shihab ad-Din Suhrawardi (d. 1191), Farid ad-Din Attar, and Jalal ad-Din Rumi, and the subsequent development of Persian, Turkish, and Urdu mystic poetry. Important Sufi figures elsewhere in the Islamic world include Muin ad-Din Chishti in India and Baha ad-Din Naqshband (d. 1390) in central Asia.

Sufi orders, which assimilated aspects of native religious traditions more readily than more dogmatic versions of Islam, played a major role in the expansion of Islam into sub-Saharan Africa and central, S, and SE Asia. The oldest extant order with attested historicity is probably the Qadiriyya, founded by Abd al-Qadir al-Jilani (d. 1166) in Baghdad. Other important orders include the Ahmadiyya (notably in Egypt), Naqshbandiyya (Central Asia), Nimatullahiyya (Iran), Rifaiyya (Egypt, SW Asia), Shadhiliyya (N Africa, Arabia), Suhrawardiyya and Chishtiyya (S and central Asia), and Tijaniyya (N and W Africa).

The work of Idries Shah was instrumental in introducing Sufism to the West; see his The Sufis (1964) and The Way of the Sufi (1968). Although Sufism has made significant contributions to the spread of Islam and the development of various aspects of Islamic civilization (e.g., literature and calligraphy), many conservative Muslims disagree with many popular Sufi practices, particularly saint worship, the visiting of tombs, and the incorporation of non-Islamic customs. Consequently, in recent centuries Sufism has been a target for Islamic reformist and modernist movements.