The monograph of Doctor of Historical Sciences K. A. Averyanov is dedicated to the life of St. Sergius of Radonezh - one of the most prominent figures in Russian medieval history. Perhaps, not so much has been written about any of the Russian saints as about the founder of the Trinity-Sergius Lavra. Nevertheless, even at the first acquaintance with this literature, a strange and surprising picture is revealed: at first glance, we know almost everything about the life of Sergius of Radonezh - and at the same time almost nothing, not even the exact dates of his life. The 14th century in the history of Rus' is extremely difficult for the historian, primarily due to the extreme paucity of sources. Scattered fragments of early chronicles as part of later chronicles, a few acts, several literary monuments - that, in essence, is all that a researcher of this time has at his disposal. At the same time, there is a whole layer of sources that are waiting for their researcher. We are talking about the lives of ancient Russian saints. The main difficulty is to be able to discern in them the features of the real historical reality of that time, to see specific historical facts. Thanks to a thorough analysis and comparison of various sources, the author for the first time managed to recreate the complete biography of the most famous Russian saint.