In the West, Justinian`s political authority never went further than parts of the Italian and Hispanic peninsulas. In the legal texts published by the Germanic kings, however, the influence of the early Eastern Roman codes on some of them is quite recognizable. In many early Germanic states, Roman citizens were still governed by Roman law for some time, even though members of the various Germanic tribes were governed by their respective codes. Roman law formed the basis of civil law used today in continental Europe and throughout Latin America. The common law, the other important legal body used in the world, developed in British courts in the Middle Ages and later spread to the United States and Commonwealth member states. The Justinian Codex and the institutes of Justinian were known in Western Europe and, together with the earlier codex of Theodosius II, served as a model for some Germanic legal systems; However, the digestible part was largely ignored for several centuries until a manuscript of the digests was rediscovered in Italy around 1070. This was done mainly through the work of glossaries, which wrote their comments between the lines (glossa interlinearis) or as marginal notes (glossa marginalis). From that point on, scholars began to study ancient Roman legal texts and teach others what they had learned from their studies. The center of these studies was Bologna. The Faculty of Law gradually developed to become the first university in Europe. The Twelve Tables became of limited use when legal issues that they did not cover arose, for example: As commercial activity has grown, it has become necessary to legally cover transactions and transactions between citizens and non-citizens and to enact laws that take into account the behavior and intentions of the parties involved.
These relations became the center of treaties and provisions such as a stipulatio, and from about 242 BC. It was presided over by a special judge (praetor peregrinus) who dealt specifically with foreign disputes and relations between Rome and foreign states, i.e. international law (ius gentium.dem i.e. international law. Rei vindicatio is an action by which the plaintiff demands that the defendant return something that belongs to him. It may only be used if the plaintiff is the owner of the thing and the defendant somehow interferes with the possession of the thing by the plaintiff. The plaintiff may also initiate an actio furti (personal action) to punish the defendant. If the case could not be restored, the plaintiff could claim damages from the defendant using condictio furtiva (a personal action). With the help of the actio legis Aquiliae (personal action), the plaintiff was able to claim damages from the defendant. Rei vindicatio was derived from ius civile and was therefore only available to Roman citizens.
An important element of Roman law was jurists (iurisprudentes), legal experts who subjected laws, rules, and written institutions to intellectual examination and discussion in order to extract the fundamental legal principles they contained, and then applied and tested these principles to hypothetical special cases and then applied them to new laws. Jurists were an elite corps, as there were probably fewer than 20 at any given time and their qualification for the role was their in-depth knowledge of the law and its history. In imperial times, they were incorporated into the general bureaucracy that served the emperor. Jurists also had a kind of monopoly on legal knowledge, as the opportunity to study law as part of the usual curriculum was not possible until the middle of the 2nd century AD. Jurists also wrote legal treatises, one of the most influential being On Civil Law (De Iure Civili) by Q. Mucius Scaevola in the 1st century BC. Roman laws covered all facets of daily life. They dealt with crime and punishment, land ownership and property, trade, maritime and agricultural industry, citizenship, sexuality and prostitution, slavery and commission, politics, liability and property damage, and peacekeeping. We can study these laws today through ancient legal texts, literature, papyri, wax tablets and inscriptions. When the center of the empire was moved to the Greek East in the 4th century, many legal terms of Greek origin appeared in official Roman legislation.
 Influence is also visible in personal or family law, which is traditionally the part of the law that changes the least. For example, Constantine began to limit the ancient Roman concept of patria potestas, the power of the male head of the family over his descendants, by recognizing that the people in the potestate, the descendants, could have property rights. He apparently made concessions to the much stricter concept of paternal authority under Greek Hellenistic law.  The Codex Theodosianus (438 AD) was a codification of constant laws. Later, the emperors went even further until Justinian finally decreed that a child of Potestate would become the owner of everything he had acquired, unless he had acquired something from his father.  In the Republic, the emphasis was more on adapting existing laws through magistrates (ius honorarium) than on creating entirely new legislation. This was done especially in the annual Praetorian Edict (codified from 131 AD), when the types of admissible cases, defence and exceptions were described and an assessment of the previous year`s legal policy was made, making the necessary legal changes accordingly. In this way, it was the application of laws that could be adapted while the law itself remained unchanged, and so a series of case formulas accumulated to provide greater legal cover for the ever-changing situation of Roman society.
For example, a fine could be increased to keep pace with inflation, but the legal principle of a fine for a specific offence remained unchanged. This allowed other officials, such as governors and military courts, to “interpret” the law and apply it on a case-by-case basis based on individual circumstances. Traditionally, the first source of Roman law was the Twelve Tablets, which are preserved only as quotations in later sources. After an initiative to gather the civil laws (ius civile) of the First Republic in one place and to end the exclusive domination of the priestly and patrician classes on legal matters, a code of law for relations between citizens was drafted separately from the sacred law (ius sacrum). This document was in fact a collection of judgements concerning only the rights of citizens, since all other parties were subject to the jurisdiction of the male head of the family (pater familias), who enjoyed considerable freedom in the treatment of persons under his care, free and non-free. The lawyers performed various functions: they prepared legal opinions at the request of private parties. They advised judges in charge of the administration of justice, especially praetors. They assisted the praetors in drafting their edicts in which they publicly announced, at the beginning of their mandate, how they would exercise their functions and the formulas according to which certain procedures were conducted. Some lawyers have also held high judicial and administrative positions themselves. The first Decemvirate completed the first ten codes in 450 BC. J.-C. This is how Livy describes its origin: one of the most important sources on Roman law is the Corpus Iuris Civilis, compiled under the patronage of Justinian I and, as its name suggests, includes civil law.
One of his four books, the Massive Digest, covers all aspects of public and private law.