The church of St. Eusebius in Vercelli, a funerary and martyrial pole in the early Christian period and a cathedral since the 10th century, represents a major ecclesiastical center in northwestern Italy. It was linked to the memory of St. Eusebius, who was buried here, and thus it became the see of one of the leading dioceses in that area, as well as of a powerful episcopate and an influential canonical chapter. The ancient building underwent total remodeling between the 16th and 19th centuries, with the exception of the bell tower, which still stands most of its height in the 12th-century phase. The paper focuses on the church’s layout before the reconstructions of the Modern Age, the subject of recent reinterpretations. Through an examination of written and cartographic sources and subsisting material data, an attempt is made to recompose the long and complex architectural history of the building, with special attention to the medieval phases, which are often underestimated in the historiography.