The lecture discusses the place of reduced vowels in the Old Russian vowel system, the character of their allophonic variation, including the rule of weak and strong position, as well as representation of reduced vowels in written sources. The lecture also describes the mechanics of the loss of reduced vowels in weak and strong positions, including the role of anaptyctic vowels in this process. Furthermore, the stages of dephonologization of reduced vowels in Old Russian are discussed.