Theodore Spandounes was born in the middle of the 15th century to a family of the Greek exiles who had found shelter in Italy after the fall of Constantinople. The Spandounes family had not played any significant role in the history of the Byzantine Empire but his mother Eudotia came from the famous Kantakouzenos family. Members of the Kantakouzenos family played an important political role in Serbia until its annexation by the Ottoman Empire in 1459. Theodore established close relations with popes Clement VII and Paul III, who he advised on the Ottoman affairs. Probably around 1515, Spandounes wrote the first version of the treatise On the Origin of the Ottoman Emperors. In 1538 he dedicated the final version to Henry, Dauphin of France (the future king Henry II).As suggested by the title, the main objective of Spandounes’s treatise was to explain how the Ottomans rose from the humble beginnings to their current mighty status in a relatively short time. In its final version from 1538, the treatise consists of four parts, different in size, composition and content. The most original and creative part, which is also of the greatest importance to the scholars interested in the Ottoman history, is the second part. However, information concerning the history of Serbia and Hungary can only be found in the first part. A detailed analysis of Theodor’s treatise leads to the following conclusions: 1) Spandounes’s remarks concerning Hungary and Serbia are generally infrequent, and the events described were rather accidentally chosen; 2) The author pays more attention to Serbia, with which he was emotionally connected through his ancestors. The information about the genealogy of the ruling family is interesting and reliable; 3) Spandounes is barely credible in his descriptions of events from the 14th and 15th century. His accounts are tendentious and quite often false; 4) Information concerning Hungary becomes more frequent for years 1520–1538, and it is relatively credible.