Roman law is the foundation of many legal systems of the world.
- Common law was originally based on Roman law, before it developed into a tradition of its own in England, from where it expanded to the United Kingdom, apart from Scotland, to the United States, apart from Louisiana, and to most former British colonies.
- By contrast, so-called Civil law systems are more directly based on Roman law; the legal systems of most countries in continental Europe and South America fall into this category, frequently through the Napoleonic Code. These are sometimes called Latin systems (or "operating jure latino").
For example, it was Roman law that developed the differentiation between contract and tort; previously (as in ancient Greek law), contract violations were simply a kind of tort. Also, the differentiation between possession (which is a factual state: someone has something) and property (which is a right; later formulated as the right to do whatever one wishes with something) was developed in Roman law, most visible in the rei vindicatio, the action of the owner against the possessor to release a piece of property. Finally, the origins of today's concept that contracts are valid when there is a meeting of the minds can be found in the Roman rules about credits, which could be freely agreed on and were called stipulatio.
Roman law also developed the concepts of one law for the citizens and another law for foreigners – the beginnings of private international law.
The Emperor Justinian arranged for the re-organisation of most of Roman law in his Codex and his Pandectae, a fifty book set which took three years to compile and was completed in 533. The emperor also ordered the production of a textbook, Iustiniani Institutiones (the Justinian teaching manual), during the early 530s. It was intended as an overview of Roman law for legal students and consisted of just four books. Justinian's work was completed by Pandectae (or Digesto), Institutiones and Codex are part of the Corpus Juris Civilis. This has been called the most influential law work ever written as it has been on the reading list for legal students in countries using Civil law for nearly 1500 years so far.
See also: List of Roman laws