Formal teaching at Najaf is usually traced to the scholar Shaykh al-Tusi, who relocated there in the eleventh century and helped establish the city as a center of Shia jurisprudence and theology independent of the Abbasid court at Baghdad. Over the following centuries Najaf's hawza developed its own distinctive methods of legal reasoning, training the scholars who would go on to staff seminaries and mosques across the wider Shia world, from Lebanon to South Asia. Its standing rests less on any single building than on an unbroken chain of teacher and student relationships stretching back nearly a thousand years, which many within the tradition regard as giving it a seniority that younger centers, including Qom, have not matched.
The city's marja al-taqlid, senior clerics recognized as authoritative sources of religious guidance, have historically held that position through scholarly reputation earned inside this teaching lineage rather than through any formal appointment, a structure that leaves real authority distributed among whichever living scholars the wider community chooses, at any given time, to follow.
Najaf was also where Twelver Shia jurisprudence settled one of its own longest running internal disputes, between the Usuli school, which holds that qualified jurists may exercise independent legal reasoning, and the Akhbari school, which favored relying directly on the sayings of the Imams. Scholars based in Najaf, including Muhammad Baqir Behbahani in the eighteenth century, argued the Usuli position to eventual dominance across nearly the whole Shia world, a shift that gave the marja system much of its later authority to issue independent rulings.