Piacenza lies on the right bank of the river Po, at a crucial crossroads in the south-west area of the Po Valley. The first settlements date back to the stone and bronze ages. Gauls and Etruscans are likely to have settled in the area at a later stage, but there are no certain traces left.

The earliest urban settlement may be traced back to the year 218 B.C., when six thousand Romans founded the colony of "Placentia". They left their mark in the layout of the town, which has a square plan organized around two main intersecting streets, called cardo (north to south) and decumanum (east to west), and a web of side streets at right angles to each other. The strength of the colony was first put to the test in the second Punic War, when the Romans fought a fierce battle against Hannibal on the river Trebbia.

In republican and imperial times, Piacenza became an important centre with a flourishing river port, and from 187 B.C. it was the end town of the Via Aemilia, a main road at the foot of the Appennines, which joined the Rome-bound Via Flaminia at Rimini.
With the beginning of the Christian era, the community consecrated modest chapels to its first martyrs, which later grew into important churches. During the Middle Ages the town was ravaged quite a few times, fell under the power of the Germanic invaders and suffered the effects of the war between the Goths and the troops of the Eastern Roman Empire. After being under the rule of Ostrogoths and Byzantines, Piacenza became the administrative centre of a Longobard dukedom, but its true recovery started in the ninth century, under the Franks.

Around the year 1000 the town entered an era of demographic, political and economical renaissance, in which a great role was played by its strategic location along the Via Francigena, at a crossroads of several important routes from the Alps to Rome, with their constant stream of merchants and pilgrims. In this age dominated by feudal lords and count-bishops, an enterprising class of merchants and craftsmen arose alongside the ancient aristocracy, representing a new financial power which, centuries later, was to turn Piacenza into one of the leading centres of Europe.
The end of the year 1000 saw the resurgence of pro-Papal sympathies: it was not by chance that Urban II chose Piacenza to proclaim the First Crusade for the liberation of the Holy Land in 1095. Piacenza became a free city in 1126, and took sides with the Lombard League against Barbarossa, who signed here the preliminary agreements for the Peace of Constance (1183).
In the twelfth and thirteenth centuries trade and commerce increased (particularly the production of textiles), agriculture flourished and the economy was boosted by the exchange fair.
Churches and monasteries were built, often with hospices annexed. It was in this period that the two architectural emblems of the town were erected: first the Cathedral, and later the Palazzo Gotico.
In the Middle Ages, however, Piacenza lived through a turbulent time of political feuding. From the second half of the thirteenth century onwards, the town and territory came in turn under the rule of the Scotti and Pallavicino families, of Alberto Scoto (1290-1313), a merchant and powerful banker, and of the houses of Visconti(until 1447) and Sforza (until 1499).
Louis XII of France, who had claims over the Milan area, ruled Piacenza until 1521.
In 1545, Pope Paul III Farnese conferred the newly-created dukedom of Parma and Piacenza on his son Pier Luigi, the first of the eight Farnese dukes who were to rule the town until 1731.
The Bourbon succeeded the Farnese until 1859, but the dukedom fell at intervals under the power of the Austrians, the French, and Napoleon, and was governed by Maria Luigia of Austria between 1816 and 1847.

In 1848, Piacenza was the first town in Italy to vote unanimously to join the kingdom of Sardinia.
In 1859, Austrian troops left the town once and for all, but the spirit of revolutionary independence was very much kept alive, as proved by the mass enlistment in Garibaldi's expedition.

The first railway bridge, opened on 3 June 1865 in the presence of the future King Umberto I, drastically improved the connections between North and South.
In 1891, the charter of the first Italian workers' association was signed in Piacenza, an early successful step in the workers' fight for protection and emancipation.

Piacenza paid a high toll for its remarkable participation in the two World Wars: more than 600 soldiers died in the first conflict, and over 4,000 died or went missing during the second.
In 1996, the city was awarded the Gold Medal for military valour by President Scalfaro in recognition of the population's strong commitment to the liberation fight against Nazism and Fascism.
Partisan brigades entered Piacenza, freed at last from the occupiers, on the morning of 28 April 1945, a day which marks the beginning of the post-war period for the town.