The Council was held at the end of Pope Urban II's tour of Italy and France, which he made to reassert his authority after the investiture controversy with the Holy Roman Empire. Two hundred bishops attended, as well as 4000 other church officials, and 30,000 laymen; there were so many people that the council had to be held outside of the city. The massive number of attendees reflects the increased authority of the church in the wake of Pope Gregory VII.
Among the lay attendees were Bertha, countess of Maurine, the wife of emperor Henry IV, who came to complain about her husband's affairs. Also in attendance were ambassadors from Philip I of France, who came to appeal Philip's recent excommunication over his illegal divorce and remarriage to Bertrade de Montfort: Philip was given until Pentecost to rectify his situation. The rest of the business of the council expressed fairly typical church concerns: there were at least 15 canons published during the council, including a condemnation of the Berengerian heresy; a condemnation of the Nicolaitan heresy; an affirmation of the presence of Christ in the Eucharist; denunciations of the Antipope Clement III and his supporters; and a prohibition of payment to priests for baptisms, burials, or confirmations.
In hindsight, the most important attendees were the ambassadors sent by Byzantine emperor Alexius I Comnenus. Alexius had been excommunicated by Gregory VII, and been through a series of reinstatements in the Church, but Urban had ultimately lifted the excommunication when he became pope in 1088, and relations between the east and west were at least temporarily friendly. The Byzantine Empire had lost much of its territory in Asia Minor to the Seljuk Turks in the aftermath of the Battle of Manzikert in 1071, and Alexius hoped western knights could help him restore it.
The ambassadors probably exaggerated the immediate danger to the empire, which was not so great, now that the Seljuks were fighting amongst themselves; Alexius also told them to remind them that Jerusalem was also held by Muslims, knowing that western Christians, too, attached a special significance to the city at the centre of the world which Muslims had denied them pilgrimage to.
Alexius' request was taken far more seriously than he had hoped. Urban may already have been thinking about a crusade to the east, and the request was interpreted as a sign of weakness in both the Eastern empire and the Orthodox church. If Urban sent help, perhaps he could also reunite the churches under his authority. News of the threat to the empire and the supposed threat to Jerusalem spread throughout France after the council ended; in November of 1095, Urban called an even bigger council, the Council of Clermont, where the organization of the First Crusade was formally announced.
Most of the information about the Council of Piacenza comes from the chronicler Bernold of Constance, who was probably there himself, as well as Ekkehard of Aura and Guibert of Nogent, who were at Clermont if not at Piacenza. No contemporary Byzantine sources felt the ambassadors were important enough to mention, but the council is mentioned by the 13th century chronicler Theodore Scutariotes, who quotes now-lost contemporary works.