[2] He later taught jurisprudence for some years, and his students included notable figures such as Cardinals Alexander Farnese, Reginald Pole and Charles Borromeo.

He had an illegitimate son after an affair with Maddalena Fulchini, Giacomo Boncompagni, but before he took holy orders, making him the last Pope to have left issue.

[3] At the age of 36 he was summoned to Rome by Pope Paul III (1534–1549), under whom he held successive appointments as first judge of the capital, abbreviator, and vice-chancellor of the Campagna e Marittima.

Here Ugo confirms his paternity of Giacomo (or Jacopo) Boncompagni (1548–1612) by Maddalena de' Fucchinis, a servant in the employ of his sister-in-law Laura Ferro.

The future Pope explains in detail the circumstances of the boy's conception, which took place in 1547 in Bologna after the Council of Trent had moved to that city; his motive was to assure his inheritance rights following the death (in 1546) of his father Cristoforo Boncompagni.

Pope Gregory XIII also founded numerous seminaries for training priests, beginning with the German College at Rome, and put them in the charge of the Jesuits.

In 1575, he gave official status to the Congregation of the Oratory, a community of priests without vows, dedicated to prayer and preaching (founded by Saint Philip Neri).

Gregory also transformed the Dominican studium founded in the 13th century at Rome into the College of St. Thomas in 1580, as recommended by the Council of Trent.

Pope Gregory XIII is best known for commissioning the Gregorian calendar, initially authored by the physician/astronomer Aloysius Lilius[7][8] and aided by Jesuit priest/astronomer Christopher Clavius, who made the final modifications.

He encouraged the plans of Philip II to dethrone Elizabeth I of England (reigned from 1558 to 1603), resulting in English Protestants suspecting Catholics as potential traitors and subversives.

[11] After the St. Bartholomew's Day Massacres of Huguenots in France in 1572, Pope Gregory signaled his approval and celebrated a Te Deum mass.

A commemorative medal was issued with Gregory's portrait and on the obverse a chastising angel, sword in hand and the legend UGONOTTORUM STRAGES ("Overthrow of the Huguenots").

Gregory also helped his son to become a powerful feudatary through the acquisition of the Duchy of Sora, on the border between the Papal States and the Kingdom of Naples.

This measure enriched his treasury for a time, but alienated a great body of the nobility and gentry, revived old factions, and created new ones, and caused economic and social chaos in the Papal States.

But a sudden change on 10 April saw him confined in his bed and it was observed that he had a cold sweat and a weak pulse; he received Extreme Unction moments before he died.