CSEL 109 is the first critical edition of the five books of Vigilius of Thapsus’ Contra Eutychetem based on all known codices. Composed in the aftermath of the Council of Chalcedon, the books deal with the refutation of the Monophysite heresy and the defence of Chalcedonism. The introduction presents the stemmatic classification of the manuscripts, discusses linguistic peculiarities and explains the reasons for textual decisions.