St. Apostle Andronicus had a special meaning for the mission of Sts. Methodius and Cyril and the evangelization of Slavs as it is obvious from the Life of St. Methodius and the story about the beginning of Slavonic writing mentioned in the Tale of Bygone Years, where he is titled as a Pannonian bishop. The New Testament and Early Fathers of the Church do not give us any information about the Pannonian bishopric of St. Andronicus. The hagiographic tradition that St. Andronicus was the bishop of Pannonia (but not of Sirmium!) was formed in the 7th – 8th centuries. It is reflected at first in so called “Apostolic catalogues” of Pseudo-Epiphanius and Pseudo-Hippolitus. The reading “bishop of Spain” in the Apostolic catalogue of Pseudo-Dorotheus can be a result of mistakes of scribes, because it is not supported by any tradition. Synaxarium of Constantinople mentions St. Andronicus as a bishop of Pannonia in a number of memorial rubrics (30 June, 30 July). According to Synaxarium, St. Andronicus and Junia crushed Pagan shrines and built Christian Churches, expelled impure spirits from men and healed incurable diseases. The information about healings by St. Andronicus and Junia may be connected with the placement of their relics in the Quarter of Eugenius in Constantinople, which, according to our opinion, happened in 610. Such precise date is connected with the mention about Patriarch Thomas, apparently Thomas the First (607 – 610), and with our hypothesis, that originally the source of Synaxarium had the name of Emperor Heraclius (610 – 641) ΗΡΑΚΛΙΟΥ, which, as a result of metathesis РА and АР and mixture of lambda and delta in uncial writing, became the name of Emperor Arcadius (ΑΡΚΑΔΙΟΥ). The later attribution of wonder-working relics as those of St. Apostle Andronicus, together with the baptism of the Serbs and the Croatians in time of Heraclius, led to the veneration of St. Andronicus as a bishop of Pannonia and, correspondingly, the miracles wrought by his relics, revealed Divine benevolence to the conversion of the Slavs. The canon in honour of St. Apostle Andronicus and Junia written by St. Joseph the Hymnographer, coinciding in a number of details with the Constantinopolitan Synaxarium, added some new information about their life. First, it tells us about the disputes with Pagan orators; second, it hints at their death as martyrs. On the other hand, it is silent about the Pannonian bishopric of St. Andronicus. This may be connected both with a variant of the text of the Life of St. Andronicus and Junia, which has not reached us, and with a certain distance of St. Joseph the Hymnographer from the mission of Sts. Cyril and Methodius, in value of his adherence to the party of St. Ignatius, the Patriarch of Constantinople. Combining the above-mentioned Greek and the Slavonic sources (the Life of St. Methodius and the Tale of Bygone Years), we can reconstruct the lost Life of St. Andronicus and Junia formed by the 9th century in the following way. They were constant followers of St. Paul and they walked around the world. St. Andronicus and Junia crushed Pagan temples and built Christian Churches, expelled demons and healed the sick. St. Andronicus accompanied St. Paul in his missionary travel to Illyricum, he reached with the latter Morava and taught Slavs. St. Andronicus and Junia died as martyrs. As a whole, this Vita, coinciding in a number of details with the Lives of other Apostles has some original features. It could have been used by Sts. Cyril and Methodius as an apostolic foundation of their mission.