The Agnelli family, who founded and still control the firm, are people with whom Italians love to identify - wealthy but not pretentious, jet-setting but not vulgar, and very, very rich. The fact that Fiat owns such national treasures as Ferrari and Juventus merely adds to the family's popularity.
Fiat is both a symbol of Italian design and success abroad, and a pillar of the national economy. The company first started turning out motor vehicles from its plant in Turin 38 years after the birth of the Italian state and the fortunes of the two have been entwined ever since. The Prime Minister, Massimo D'Alema, and the recently elected President, Carlo Azeglio Ciampi, will be among the 3,000 guests at a gala banquet tomorrow night at Fiat's headquarters, the Lingotto building.
The first Fiat plant opened with 35 workers but it was a visit the firm's founder, Giovanni Agnelli, paid to Ford's Detroit works in 1912 that inspired the decision to introduce mass production to Italy. The head of Fiat today is Giovanni's grandson, 78-year-old Gianni Agnelli.
Like many of Italy's most successful companies Fiat remains solidly under family control; Gianni Agnelli's designated successor as company head was his handsome and charming nephew Giovannino. However, he died of stomach cancer last year at the age of 33.
Fiat's influence on national politics is immense. Despite or maybe because of its political muscle Fiat emerged more-or-less unscathed from the clean- hands anti-corruption inquiry that wiped out many of Italy's political and business leaders in the early Nineties.
Even more symbiotic, though, is the relationship between the automotive empire and the city of Turin.
The veteran journalist Giorgio Bocca, writing in La Repubblica recently, recalled. "For decades Fiat has been the real administrator of the city and the guide of the Piedmont region; the town council fitted around its working day, the public services followed the factory shifts, mayors and police chiefs all obeyed the telephone calls from Corso Marconi. It was hard to tell where servility finished and civic and company patriotism started."
Bocca noted that Fiat - like Italy - had changed more in the past decade than in the previous nine. Its employees, greatly reduced by the introduction of new technology, are no longer the champions of the Italian working class. Only about half of Fiat's output comes from Turin, the remainder from 13 plants around the globe.
The centenary celebrations come as a welcome diversion from concerns over Fiat's future. The anomaly of Fiat is that as an automobile empire it is too big to be gobbled up by a rival yet not big enough to take over another of its competitors. With increasing moves towards concentration of ownership in the car industry, Fiat's future may depend on the dilution of the family shareholding that has characterised its first century of success.