He changed much of the Roman law, explaining what each law meant, named Corpus Iurus Civilus (Civil law; which nearly all legal systems base theirs off of). He also surrounded himself with an advisory, and many who were put there on their merit, not their birth circumstances. The Western Roman Empire was collapsed due to terrible leadership, Inflation, and frequent barbarian raids. Justinian tried to take back the empire.  He took back North Africa, Southern Hispania, the coastal regions Illyria, and Italia. He also started expensive campaigns to rebuild Italia. Many historians today would call him a dreamer as he wished to reclaim the entire empire, would pay for themselves. 

The cash reserves in the empire were practically empty after his death due to the constant warring in Persia, North Africa, and Italy; the borders were not easy to protect, and the empire was overextended. This lead to the collapse of many parts of the Byzantine empire, and the weakening of the empire for many years to come. He also had increased the taxes tremendously to keep to pay for his conquests. There were many riots happening in Constantinople in his reign, though he did streamline many laws making the riots less frequent.

Justinian said, "frugality is the mother of all virtues", which in hindsight is very ironic as he used much of the Treasury to reclaim the empire.

Professor Freeman said, "This is a very advanced science in Roman Law, As advanced as it is anywhere, at any time.", He was praising him for him making the Justinian code.