The main sources concerning Heraclius’ first campaign against the Persians are : (a) The poem of George Pisides, known under the Latin title Expeditio Persica; Pisides, an eyewitness, composed and read his panegyric poem shortly after the end of the campaign, when the emperor returned to Constantinople.(b) A chapter of the Chronicle of Theophanes the Confessor (818) concerning the events of the year 622; the first part of this chapter, concerning the preparations and the beginning of the campaign (p. 302, l. 32-p. 303, l. 17) is based on sources that have not been preserved; the second part (p. 303, l. 17-p. 306, l. 8) depends almost completely upon the poem of Pisides. Of the other Greek sources, the Paschal Chronicle and the Chronicle of Patriarch Nicephorus do not even mention the campaign, while later chroniclers, such as Cedrenus and Zonaras, summarize and reproduce the account of Theophanes. The non-Greek chronicles have also a very limited importance for our research since most of them ignore this first campaign, or contain no chronological data.