This volume is the first in the SIR series dedicated to the work of Domitius Ulpianus, the most widely used jurist in the Digesta Iustiniani, which contains about 3000 fragments taken from his works. In addition to his studies, Ulpianus also held prestigious political positions, and was, for about a year, between 222 and 223 until his murder, perhaps the most powerful character of the empire and certainly the closest to the young emperor, Severus Alexander, of whom he was a kind of guardian. His death, in a conspiracy of praetorians, symbolically marks the end of the great Roman legal thought. This volume presents two of his works: the Institutiones and the De Censibus. The first identifies a crucial moment in the intellectual biography of its author and his political- cultural program: the construction of a model of legality in which the universal empire could be recognized, and which could act as an embankment against a despotic-military drift of which all dangers were glimpsed.