The lecture provides a general description of the sound system of late Proto-Slavic and early Old Russian periods. The vowel and consonant systems are described. The lecture also gives a summary of phonetic changes in the Proto-Slavic period, connected with the tendency towards open syllables (increasing sonority) and intrasyllabic (within a syllable) vowel harmony: loss of final consonants, simplification of consonant combinations, development of prosthetic consonants, and mutation of vowels after palatal consonants, etc.