The Code of Justinian (in Latin, Codex Justinianus) is a substantial collection of laws compiled under the sponsorship of Justinian I, ruler of the Byzantine Empire. Although the laws passed during Justinian's reign would be included, the Codex was not a completely new legal code, but an aggregation of existing laws, portions of the historic opinions of great Roman legal experts, and an outline of law in general.
Work began on the Code shortly after Justinian took the throne in 527. While much of it was completed by the mid-530s, because the Code included new laws, parts of it were regularly revised to include those new laws, up until 565.
There were four books that comprised the Code: Codex Constitutionum, the Digesta, the Institutiones and the Novellae Constitutiones Post Codicem.
The Codex Constitutionum was the first book to be compiled. In the first few months of Justinian's reign, he appointed a commission of ten jurists to review all the laws, rulings and decrees issued by the emperors. They reconciled contradictions, weeded out obsolete laws, and adapted archaic laws to their contemporary circumstances. In 529 the results of their efforts were published in 10 volumes and disseminated throughout the empire. All imperial laws not contained in the Codex Constitutionum were repealed.
In 534 a revised codex was issued that incorporated the legislation Justinian had passed in the first seven years of his reign. This Codex Repetitae Praelectionis was comprised of 12 volumes.
The Digesta (also known as the Pandectae) was begun in 530 under the direction of Tribonian, an esteemed jurist appointed by the emperor. Tribonian created a commission of 16 attorneys who combed through the writings of every recognized legal expert in imperial history. They culled whatever they though was of legal value and selected one extract (and occasionally two) on each legal point. They then combined them into an immense collection of 50 volumes, subdivided into segments according to subject. The resulting work was published in 533. Any juridical statement that wasn't included in the Digesta was not considered binding, and in future it would no longer be a valid basis for legal citation.
When Tribonian (along with his commission) had finished the Digesta, he turned his attention to the Institutiones. Pulled together and published in about a year, the Institutiones was a basic textbook for beginning law students. It was based on earlier texts, including some by the great Roman jurist Gaius, and provided a general outline of legal institutions.
After the revised Codex was published in 534, the last publication, the Novellae Constitutiones Post Codicem was issued. Known simply as the "Novels" in English, this publication was a collection of the new laws the emperor had issued himself. It was reissued regularly until Justinian's death.
With the exception of the Novels, which were almost all written in Greek, the Code of Justinian was published in Latin. The Novels also had Latin translations for the western provinces of the empire.
The Code of Justinian would be highly influential through much of the Middle Ages, not only with the Emperors of Eastern Rome, but with the rest of Europe.