Justinian I (Flavius Anicius Justinianus), the nephew of Justin I, was born at Tauresium in Illyria, the son of a Slavonic peasant, and was originally called Sabbatius. Educated at Constantinople, in 521 Justinian was named consul and in 527 was proclaimed by Justin his colleague in the empire. Justin died the same year and Justinian, who was then proclaimed sole emperor, was crowned along with his wife, Theodora.

Justinian had the good sense to select the most able generals and under Narses and Belisarius his reign can be said to have restored the Roman Empire to its ancient limits, and to have reunited East and West. His first war (with Persia) ended with a treaty favorable to Justinian. But in 532 came the Nika Riots, an outburst of political turmoil that went as far as to elect a rival emperor. Justinian considered fleeing Constantinople but thanks to Narses, Belisarius and Theodora, the riots were contained (35,000 rioters were killed in one day). Through Belisarius, the Vandal kingdom of Africa as reannexed to the empire and Belisarius and Narses restored the imperial authority in Rome, Northern Italy and Spain.

At great cost, Justinian constructed a vast line of walls along the eastern and southeastern frontier of the empire. But it was as legislator that Justinian gained his fame. He collected and codified all the principal imperial statues. The Codex, by which all previous imperial enactments were repealed, was published in 529. The writings of the jurists were next published as the Digest of 533. Finally, there appeared in the same year, the Institutiones, a systematic and elementary treatise on the law. In later years, Justinian promulgated several new laws, known as the Novellae. The Institutes, Digest, Code and Novels together comprise what is known as the Corpus Juris Civilis, the Body of Civil Law.

A selection from Justinian's Institutiones is included here.

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PREAMBLE
In the Name Of Our Lord Jesus Christ.
THE EMPEROR CAESAR FLAVIUS JUSTINIANUS, VANQUISHER OF THE ALAMANI, GOTHS, FRANCS, GERMANS, ANTES, ALANI, VANDALS, AFRICANS, PIOUS, HAPPY, GLORIOUS, TRIUMPHANT CONQUEROR, EVER AUGUST, TO THE YOUTH DESIROUS OF STUDYING THE LAW, GREETING.
The imperial majesty should be not only made glorious by arms, but also strengthened by laws, that, alike in time of peace and in time of war, the state may be well governed, and that the emperor may not only he victorious in the field of battle, hut also may by every legal means repel the iniquities of men who abuse the laws, and may at once religiously uphold justice and triumph over his conquered enemies.
1. By our incessant labours and great care, with the blessing of God, we have attained this double end. The barbarian nations reduced under our yoke know our efforts in war; to which also Africa and very many other provinces bear witness, which, after so long an interval, have been restored to the dominion of Rome and our empire, by our victories gained through the favour of heaven. All nations moreover are governed by laws which we have either promulgated or arranged.
2. When we had arranged and brought into perfect harmony the hitherto confused mass of imperial constitutions, we then extended our care to the endless volumes of ancient law; and sailing as it were across the mid ocean, have now completed, through the favour of heaven, a work we once despaired of.
3. When by the blessing of God this task was accomplished, we summoned the most eminent Trihonian, master and ex-quaestor of our palace, together with the illustrious Theophilus and Dorotheus, professors of law, all of whom have on many occasions proved to us their ability, legal knowledge, and obedience to our orders; and we specially charged them to compose, under our authority and advice, Institutes, so that you may no more learn the first elements of law from old and erroneous sources, but apprehend them by the clear light of imperial wisdom; and that your minds and ears may receive nothing that is useless or misplaced, but only what obtains in actual practice. So that, whereas, formerly, the foremost among you could scarcely, after four years’ study, read the imperial constitutions, you may now commence your studies by reading them, you who have been thought worthy of an honour and a happiness so great as that the first and last lessons in the knowledge of the law should issue for you from the mouth of the emperor.
4. When therefore, by the assistance of the same eminent person Tribonian and that of other illustrious and learned men, we had compiled the fifty books, called Digests or Pandects, in which is collected the whole ancient law, we directed that these Institutes should be divided into four books, which might serve as the first elements of the whole science of law.
5. In these books a brief exposition is given of the ancient laws, and of those also, which, overshadowed by disuse, have been again brought to light by our imperial authority.
6. These four books of Institutes thus compiled, from all the Institutes left us by the ancients, and chiefly from the commentaries of our Gaius, both from his Institutes and his Journal, and also from many other commentaries, were presented to us by the three learned men we have above named. We read and examined them, and have accorded to them all the force of our constitutions.
7. Receive, therefore, with eagerness, and study with cheerful diligence, these our laws, and show yourselves persons of such learning that you may conceive the flattering hope of yourselves being able, when your course of legal study is completed, to govern our empire in the different portions that may be entrusted to your care.
Given at Constantinople on the eleventh day of the calends of December, in the third consulate of the Emperor Justinian, ever August.

BOOK ONE
Justice is the constant and perpetual wish to render every one his due.
1. Jurisprudence is the knowledge of things divine and human; the science of the just and the unjust.
2. Having explained these general terms, we think we shall commence our exposition of the law of the Roman people most advantageously, if we pursue at first a plain and easy path, and then proceed to explain particular details with the utmost care and exact ness. For, if at the outset we overload the mind of the student, while yet new to the subject and unable to bear much, with a multitude and variety of topics, one of two things will happen—we shall either cause him wholly to abandon his studies, or, after great toil, and often after great distrust of himself (the most frequent stumbling-block in the way of youth), we shall at last conduct him to the point, to which, if he had been led by an easier road, he might, without great labour, and without any distrust of his own powers, have been sooner conducted.
3. The maxims of law are these: to live honestly, to hurt no one, to give every one his due.
4. The study of law is divided into two branches; that of public and that of private law. Public law regards the government of the Roman Empire; private law, the interest of individuals. We are now to treat of the latter, which is composed of three elements, and consists of precepts belonging to natural law, to the law of nations, and to the civil law.

[Source: The Institutes of Justinian, trans. Thomas C. Sandars (London: Longmans, Green, 1874), pp. 1-7.]