The paper discusses the controversy that has arisen concerning the origin and nature of vowel harmony in Mongolian, as well as in a number of other Eurasian languages. In contrast to the conventional understanding according to which Proto-Mongolic had a palatal-velar harmony of the same type as is attested in the Turkic and Uralic languages, it has been claimed recently that the harmony was actually of the tongue root type, involving, in particular, tongue root retraction in the pronunciation of certain vowels. However, while tongue root harmony is indeed prevalent in many modern Mongolic languages, including standard Mongolian, there are several arguments that can be made in support of the conventional reconstruction. There are serious reasons to assume that Mongolic has undergone a process of vowel rotation, which has turned the originally palatal-velar harmony to tongue root harmony. In this process the originally horizontally organized harmonic pairs have become verticalized. A typical result of the verticalization has been the rapid reduction of the original vowel paradigm as well as the development of new palatal vowels to complement the losses.