This paper examines the main phonological properties of vowel harmony, with facts drawn exclusively from Uralic and Altaic languages. It is demonstrated that vowel harmony rules apply iteratively; and that exceptional stems and suffixes differ in that the former do not serve as context to the vowel harmony rule, while the latter do. Evidence is presented for the incorporation of abstract segments and absolute neutralization rules into phonological theory. Specifically, abstract and non-abstract analyses are compared side by side in a Turkish dialect, Buriat, Manchu, Vogul, Cheremis, and Hungarian: in each case the abstract analysis is shown to be superior or the only one tenable. The abstractness controversy is discussed in detail, and a principled resolution is offered.