Ranuccio I Farnese was the fourth duke of Parma and Piacenza, regent from 1586 to 1592, duke from 1592 to death and the fifth duke of Castro.
He gave the city of Parma unique monuments (the Citadel, the Pilotta and the Farnese Theater), and modern legislation, which made it a center of excellence both in lifestyle and as an architectural model. Despite its particular character, it made Parma a cultural capital at the same level as London and Paris.
Through his mother, the eleven year old Ranuccio became the legitimate heir to the throne of Portugal at the time of the extinction of the Aviz dynasty, which occurred in 1580, but his father Alessandro had to give up the crown of his son in favor of his far more powerful uncle, King Philip II of Spain.
Ranuccio was the son of the great condottiere Alessandro Farnese and Maria d'Aviz. With a gracile constitution and sickly temper, at the death of his grandfather Ottavio, at seventeen, he was in charge of the regency of the duchy. His mother's untimely death, which occurred in 1577, when Ranuccio was only eight years old, and the continued absence of his father, governor of Flanders, forced him to a solitary youth. Crescendo strengthened physically and, despite being a skilled soldier, as the family tradition wanted, he preferred to deal with law and administration. During his long government, which lasted 30 years, he reorganized the structures of the duchy, established a new balance between feudal government and power, and sought to stimulate trade and the silk and majolica industries.

In the period from 1591, the last year of regency to 1601, Ranuccio had the city walls restored, which were demolished and rebuilt for a quarter of the route. On the North-East side, four large fortified bastions were built. Also in this period he gave new impetus to the university by joining it to the Collegio dei Nobili (or Santa Caterina) and placing it under the control of the Jesuits. At the behest of his father, who, although he was in Flanders, gave precise instructions on the management of the duchy, he built a fortified citadel, inspired by that of Antwerp, he began work on the Pilotta palace (completed in 1620), which contained also the Farnese Theater (completed in 1618), capable of 4,500 seats, continued work on the Palazzo del Giardino and commissioned, for the city of Piacenza, Francesco Mochi two equestrian statues depicting his father and himself.
In 1594 he promulgated the Constitutions, a code containing exemplary measures, modern and far-sighted, which remained in force for centuries: the abolition of festive work, the prohibition of the cultivation of rice because it bears malaria, the construction of land reclamation and the creation of embankments on the rivers. Among the measures there was also the rationalization of waste disposal: the garbage, before being placed in suitable places, had to be collected in underground holes. In the constitutions it was also foreseen that new buildings could not be built without the permission of the authorities in charge and that the industries producing unpleasant smells were located in areas specially created in the remote parts of the inhabited area. On 7 May 1600, now in his thirties and having had many lovers, Ranuccio married eleven-year-old Margherita Aldobrandini, nephew of Pope Clement VIII. It seems that from this union the Farnese family began to have the problems of overweight that characterized all the descendants of Ranuccio. The marriage was not fruitful and so, in 1605, the duke recognized the natural son Ottavio who was born of one of his many relationships in 1598.
In 1606 Ranuccio reserved only for himself the right to hunt in some areas where before they could drive out all the nobles. Their discontent, however, was immediately felt through signs of subterranean rebellion, followed by retribution by the duke, especially against the Marquise of Colorno, Barbara Sanseverino, already a lover of his grandfather Ottavio.
In 1610 Margherita managed to complete a pregnancy and Alessandro was born, but the child was deaf and dumb.
In the spring of 1611 Alfonso Sanvitale, Count of Fontanellato, was arrested on charges of uxoricide. One of his servants, Onofrio Martani, subjected to torture, revealed details that brought to light an alleged conspiracy against the duke. Some relatives of the count and various nobles were arrested, including Count Orazio Simonetta and his wife Barbara Sanseverino, Pio Torelli count of Montechiarugolo, Gianfrancesco Sanvitale, Girolamo Sanvitale, Giambatista Masi and many others. They were all subjected to torture and made ample confession: they should have killed the duke and exterminated his family during a religious service.
If there is no doubt that Ranuccio had long desired to get his hands on the fertile feuds of the incriminated nobles, it is not entirely clear that the conspiracy was an invention of Ranuccio and his powerful advisor Bartolomeo Riva. In any case on May 4, 1612, the judge Filiberto Piosasco pronounced the guilty sentence for all the accused. They were guilty of lese majesty and were condemned to the confiscation of assets and be beheaded and hung quartered. The duke confirmed the death sentence, but prohibited the torture. The sentences were carried out on the morning of May 19, among a crowd that even gathered on the roofs of the houses. Seven heads were infixed to the nails of the gallows, and a long list of estates and houses added to the ducal registers. After this experience, always in 1612, the heir arrived, Odoardo. The confiscation of the goods allowed Ranuccio to transform the palace of Colorno into a small versailles and to create a real art collection. In 1620 Ranuccio informed Ottavio that his successor would be the legitimate son Odoardo, but Ottavio rebelled and his father had him locked up in the prisons of Rocchetta where he remained until his death, thus ensuring the succession to Odoardo.
Ranuccio died March 5, 1622. The regency of the duchy was entrusted first to his uncle, Cardinal Odoardo and then, at the death of these, to his mother.