When the Roman Empire in the West collapsed in 476 A.D, the Empire in the East continued on. It survived for three reasons: the population was larger and more urbanized than the West, making it more difficult to invade, its government was located at the fortress of Constantinople, which was almost impregnable, and its leaders were better at running government than those in the West. Perhaps the greatest Eastern Emperor was Justinian, who ruled from 527-565 A.D. Well educated and inclined toward the structure of law, Justinian believed it was his mission to codify the law of the Roman Empire. He engaged the greatest legalists of his time to prepare the new code based on rationality, coherence, equity, and the furtherance of imperial power. This Roman Civil law, also called The Justinian Code, was completed in 534 A.D.
Unfortunately, the new code fell into disuse with the failure of the Eastern empire. Justinian tried to re-capture Italy from the Ostrogoths to re-unite the empire, but he failed, and after his death, the link with the West was permanently broken, and the Byzantine Empire moved away from its Roman roots to became more Greek, Balkan, and oriental in character.
Six hundred years passed and the code was unused, because conditions in Europe were too primitive to see value in a legal system designed for mature governments. No one is sure how it was resurrected, but the code began to be studied in Northern Italy in the last decade of the eleventh century. Its rational qualities made it useful for academic study and, in turn, created an environment which fostered the growth of the legal profession in Europe. As it became more well known, the code began to impact the political thinking of European leaders, most notibly Frederick I of Germany, who fully embraced it in the 1160s. Frederick and other European leaders saw two benefits: they could use its structure to define the way lawyers are used in government administrations, and they could adopt its ideology as a justification of the right of kingship, based on the Roman concept of the supreme emperor.
The Justinian Code swept through Europe, influencing the legal systems of every country with the exception of England. Ahead of Europe in legal thinking, Henry I and Henry II had modified the old Germanic legal system into a code of Common Law that was strong enough to withstand the influence of the new code. The Common Law system of England became the legal system of the United States after the Revolutionary War in all states except one. Louisiana adopted the Justinian Code, because the state's legal system was established during the time when the territory was under French rule.
Today most of the world uses The Justinian Code, with the exception of England and its former colonies.