Abbasid era (750–1258)

Expansion of the Muslim world continued by both conquest and proselytism as both Islam and Muslim trade networks were extending into sub-Saharan West Africa, Central Asia, Volga Bulgaria and the Malay archipelago. The Ghaznavids and Ghurids conquered much of the Indian subcontinent. Many Muslims went to China to trade, virtually dominating the import and export industry of the Song Dynasty.

The major hadith collections were compiled. The Ja'fari jurisprudence was formed from the teachings of Ja'far al-Sadiq while the four Sunni Madh'habs, the Hanafi, Hanbali, Maliki and Shafi'i, were established around the teachings of Abū Ḥanīfa, Ahmad bin Hanbal, Malik ibn Anas and al-Shafi'i respectively. Al-Shafi'i also codified a method to establish the reliability of hadith. Al-Tabari and Ibn Kathir completed the most commonly cited commentaries on the Quran, the Tafsir al-Tabari in the 9th century and the Tafsir ibn Kathir in the 14th century, respectively. Philosophers Al-Farabi and Ibn Sina (Avicenna) sought to incorporate Greek principles into Islamic theology, while others like Al-Ghazzali argued against them and ultimately prevailed.

Caliphs such as Mamun al Rashid and Al-Mu'tasim made the mutazilite philosophy an official creed and imposed it upon Muslims to follow. Mu'tazila was a Greek influenced school of speculative theology called kalam, which refers to dialectic. Many orthodox Muslims rejected mutazilite doctrines and condemned their idea of the creation of the Quran. In inquisitions, Imam Hanbal refused to conform and was tortured and sent to an unlit Baghdad prison cell for nearly thirty months. The other branch of kalam was the Ash'ari school founded by Al-Ash'ari. Some Muslims began to question the piety of indulgence in a worldly life and emphasized poverty, humility and avoidance of sin based on renunciation of bodily desires. Ascetics such as Hasan al-Basri would inspire a movement that would evolve into Sufism. Beginning in the 13th century, Sufism underwent a transformation, largely because of efforts to legitimize and reorganize the movement by Al-Ghazali, who developed the model of the Sufi order—a community of spiritual teachers and students.

Islamic civilization flourished in what is sometimes referred to as the "Islamic Golden Age". Public hospitals established during this time (called Bimaristan hospitals), are considered "the first hospitals" in the modern sense of the word, and issued the first medical diplomas to license doctors of medicine. The Guinness World Records recognizes the University of Al Karaouine, founded in 859, as the world's oldest degree-granting university. The doctorate is argued to date back to the licenses to teach in law schools. Standards of experimental and quantification techniques, as well as the tradition of citation, were introduced to the scientific process. An important pioneer in this, Ibn Al-Haytham is regarded as the father of the modern scientific method and often referred to as the "world’s first true scientist." The government paid scientists the equivalent salary of professional athletes today. Discoveries include gathering the data used by Copernicus for his heliocentric conclusions and Al-Jahiz’s proposal of the theory of natural selection. Rumi wrote some of the finest Persian poetry and is still one of the best selling poets in America. Legal institutions introduced include the trust and charitable trust (Waqf).

The first Muslims states independent of a unified Muslim state emerged from the Berber Revolt (739/740-743). In 836, the capital was moved to Samarra by Caliph Al-Mu'tasim and it was returned to Baghdad in 892. In 930, the Ismaili group known as the Qarmatians unsuccessfully rebelled against the Abbassids, sacked Mecca and stole the Black Stone. By 1055 the Seljuq Turks had eliminated the Abbasids as a military power but continued the caliph's titular authority. The Mongol Empire finally put an end to the Abbassid dynasty, killing its last Caliph at the Battle of Baghdad in 1258.