The present study comprehensively analyzes vowel harmony as an important phonetic rule in Turkic languages. Recent changes in the vowel harmony potential of Turkic sounds caused by linguistic and extra-linguistic factors were described. Vowels in the Kazakh, Turkish, and Uzbek language were compared. The way this or that phoneme sounded in the Proto-Turkic language and the way it sounds now in each specific language was investigated. The common and distinguishing features of phonemes were specified based on concrete facts. The specificity of sounding of key vowel phonemes was analyzed with regard to the findings of prominent phoneticians. It was found that of the compared languages, only the Turkish and the Kazakh language preserved the classic eight ProtoTurkic vowels. Certain signs of vowel harmony were characterized -- tree types of vowel opposition: by backness--front, back, central, by labialization--unlablialized-labialized, by roundness--narrow, wide, and medium. Thus, it is possible to see the full picture of the phonemic spectrum of vowel functioning in the Turkic languages under consideration by distinguishing the common trends in changes at the current stage of linguistics with regard to the specificity of each individual language system.