After submitting the tribes in Arabian Peninsula and conquering new territories, Muslims needed to include new legal provisions, new legal uses and new legal norms in the existing practice of law, as long as they were considered compatible with Islam. During this first stage – a stage that also included the formation of Islamic thought -, the judicial authority did not have its own methodological framework yet, so lawyers often needed to rely upon their personal opinion (r'ay). Customs and usages were gradually Islamized. In the third century of Islam, traditionalists - specialists of the hadīth and of traditions - developed a new science (usūl al-fiqh) setting out "principles of law" meant to rationalize and legitimize jurisprudence. To be legal, a judgment had to be related to the teaching of the Prophet and his companions.
A group of experts from Iraq and Arabia undertook a major work to develop Islamic law and create a corpus of what was allowed or forbidden under religious norms and methods of legal thought. This work took place alongside the remarkable intellectual expansion under the Abbāsid Empire from the 8th century to the 12th century, in a context of sometimes violent rivalries. Small law schools were supplanted by large well-known official law schools. Each law school referred to the authority of its founder. It had its legal corpus, its methods, its own courts, its schools, its mosque or madrasa. Its jurisprudence was named the figh, a term designating both the doctrine and the methods, consigned in large corpora. All schools recognized the same legal foundations - the Quran and the hadīth. The Islamic law defined the major obligations for worship (the "Five Pillars of Islam"), the requirements relating to food, and the rules stating the relations between human beings.
The Islamic law was based on “four sources of evidence”. Al-Shāfi'ī worked on a law methodology reducing the share of personal opinion. When answers were lacking in the Holy Texts, lawyers had to seek solutions. Most major schools merely relied upon the consensus of qualified authorities (ijmā'). Depending on schools, reasoning methods - analogy (qiyās) and effort of interpretation (ijtihād) – were not given the same importance. Some schools preferred either the “best” solution or the right solution for the benefit of the community while respecting the Sunnah. Other schools merely relied upon the literal interpretation of Holy Texts.

The Shiite legal doctrine gave a major importance to the ijithād, and first of all to the ijithād of the main religious authorities. During the lifetime of these authorities, their legal treatises were considered as reference work for all spiritual and legal issues. Twelver Muslims [see Islam module I page 5] could freely choose the authority to consult (marja') for everyday life issues.

The jurisprudence and doctrine of Islamic law (fiqh) were based on the Sha'ria, a polysemous Quranic term meaning the way, the driving, and the revealed law. This "divine teaching" was both a reference and a religious norm.
Most major Sunni schools were related to a theological tradition. For example, Hanafism taught the doctrine of Māturīdī (10th century). From the 11th century to the 19th century, Ash'arism was the main doctrine officially taught in Sunni mosques and universities [see Islam module I page 4]. The revealed and sovereign Law resulted from God’s will, and God’s will was the only source of obligations, as claimed in many Quranic verses (4:64; 4:71, etc.).
Muslim judges punished the violations of law and defined some categories: legal/illegal, compulsory/prohibited, recommended/reproved, etc., for example for food regulations on the status of species and the killing of animals for human consumption, with stringent requirements. Believers had to ensure their body integrity by avoiding impure foods. However, the Quran only mentioned one illegal meat (haram), the meat of pork, and the tradition is unclear for many other animals (hedgehog, dog, cat, donkey, camel, etc.). On these main food issues, important for everyday life, law schools adopted nuanced positions. Thus, the consumption of horses, considered as unlawful, was reprehensible only in Malekism. Any fish was lawful (halāl) in all schools attended by Sunnis, whereas only fish with fins and scales was considered as lawful by Imāmis.

The ijithād has again become an essential tool in present relations between religion and modernity. All Muslim states have seen a major evolution in their laws since the early 20th century. Many laws have distanced themselves from the sharī'a. “Modern” civil codes have been adopted in most Arab countries. However, regarding family law and personal status, resistance has remained strong, particularly for women. Some Muslim States have developed within the context of "Islamic law" a new legislation on family, inheritance, marriage contracts, and even criminal law. In Europe too, there are tensions between norms and daily life, and between the official religious discourse and the practices of law interpreters. Islamic jurisprudence is increasingly split between rival groups. To regulate this, a committee of 33 specialists, the CEFR (European Council for Fatwā and Research - Conseil Européen pour la Fatwâ et la Recherche), was established in 1997, with at least two thirds of its members residing in Europe. The CEFR has issued its own fatwās (reasoned opinions) as legal tools to reassert the obligations of Muslims and to confer some legitimacy to the unavoidable evolution of behaviours in modern societies.