Trevi (Italian: [ˈtrɛːvi]; Latin: Trebiae) is an ancient town and comune (municipality) in Umbria, Italy, on the lower flank of Mount Serano [it] overlooking the wide plain of the Clitunno river system.

[3] The population of the comune was c. 8,000 in 2004, with the town proper accounting for about half of that; the rest lives in the frazioni (boroughs) of Borgo, Bovara, Cannaiola, Coste, Pigge, Manciano, Matigge, Parrano, Picciche, San Lorenzo and Santa Maria in Valle.

The mountainous area is now witnessing a considerable depopulation — as elsewhere in Italy, in the second half of the 20th century settlements at the highest elevations have been rapidly losing their inhabitants to the plains.

Their courses are by now highly artificial, the result of land and water management projects undertaken over many centuries, since they are recorded at least as early as the time of Theodoric the Great (6th century) and have continued down to our own time with the construction of dams and works to regularize the seasonal waters of the Marroggia that had been subject to frequent torrential overflows with sudden and disastrous results.

The most important of the year-round watercourses is the Clitunno River, celebrated in antiquity as the Clitumnus, whose deified waters were reputed to have miraculous properties and which have been lauded in prose and verse by Pliny the Younger, Propertius, Claudian, Addison, Byron and Carducci.

After the earliest period, the history of which is essentially unknown, but to which the walls in the core hill portion of the town attest, dated to the 1st century BC, the first stage of the development of Trevi beyond the hill took place under the Empire, when Hadrian restored the main road through the territory, the Via Flaminia, thus spurring the growth of a suburb in the plain at the place now called Pietrarossa, where sporadic excavations over several centuries have brought to light many remains: among them Roman baths that appear to have been still more or less in use in the time of St. Francis, who is known to have visited the area and to have advised people to bathe there.

The comuni of Trevi and Spoleto are known for the quality of their oil, a result of near-ideal calcareous soil with excellent drainage, just the right altitude for the cultivation of olive trees, and favourable climatic conditions on the west-facing lower slopes of the Apennine mountain range.

The patron saint of Trevi is St. Emiliano; his feast is celebrated on January 27 with a night-time procession of the Illuminata, in which his statue is carried out of the Duomo around the city along the line of the earliest medieval walls.