Medieval Roman law is the continuation and development of ancient Roman law that developed in the European Late Middle Ages. Based on the ancient text of Roman law, the Corpus iuris civilis, it added many new concepts, and formed the basis of the later civil law systems that prevail in the vast majority of countries. Some exceptions are, for example, common law countries of the English-speaking world
Although some legal systems in southern Europe in the Early Middle Ages, such as the Visigothic Code, retained some features of ancient Roman law, the main texts of Roman law were little known until the rediscovery of the Digest in Italy in the late 11th century. It was soon apparent that the Digest was a massive intellectual achievement and that the assimilation of its contents would require much time and study. The first European university, the University of Bologna, was set up in large part with the aim of studying it.
The ancient Roman law texts were not very explicit about matters of principle, and the commentators found it necessary to develop the scholastic method of comparing potentially conflicting texts and inferring principles that would explain the apparent contradictions. The commentators of the 12th and early 13th centuries, called glossators, such as Azo of Bologna and Accursius, produced large-scale harmonization of and commentary on the texts. They developed new concepts by reflecting on different related texts, such as the concept of half-proof in the law of evidence. Many of these glosses or interpretations, were compiled into one text around 1220 by the famous glossator Franciscus Accursius. This text was known as the Glossa Ordinaria and was a compilation of the most important commentaries made by the glossators.
Initially the rediscovered Roman law was not the law of any particular country or institution, but as lawyers trained in the concepts of Roman law came to dominate the legal profession, Roman law came to have an immense effect on law as actually practiced. For example, torture was reintroduced into Europe as a means of acquiring evidence, usually when there was half-proof or more against a defendant but not yet sufficient proof for conviction.
English law incorporated a substantial amount of Roman concepts through the works of Glanvill and Bracton. But it adopted the non-Roman jury system as the main form of evidence evaluation, thus remaining less influenced by Roman law than continental systems. However, some English courts, such as the Court of Admiralty, operated on Roman law principles.
The postglossators of the 14th century, such as Bartolus de Saxoferato and Baldus de Ubaldis, developed a more mature and deeper legal theory, less closely tied to the ancient texts. That gave the law sufficient flexibility to incorporate new concepts. For example, the first printed book on insurance was the Roman law treatise On Insurance and Merchants' Bets by Pedro de Santarém (Santerna), written in 1488 and published in 1552.
Roman law often acted (except in England) as a "common law" (ius commune) that filled the gap where local laws were silent, as well as supplying principles of interpretation for those laws in doubtful cases. In this way, Scots law was heavily influenced by Roman law from the 15th century.
Roman law was in part incorporated in later codifications of continental law such as the Napoleonic Code and hence formed a core of their successors, the civil law systems of modern European and other countries.
Roman law also had wide influence on Western political theory. Questions such as the scope and limits of government and the permissibility of tyrannicide were seen in legal terms and discussed by writers whose primary training was in law. These ideas formed the basic of modern constitutionalism, the idea that government should be bound by fixed legal norms.