Vowel harmony is a type of long-distance assimilatory phonological process involving vowels that occurs in some languages. A vowel or vowels in a word must be members of the same subclass (thus "in harmony"). In languages with vowel harmony, there are constraints on which vowels may be found near each other. Suffixes and prefixes will usually follow vowel harmony rules. Many agglutinative languages have vowel harmony.The term vowel harmony is used in two different senses.

In the first sense, it refers to any type of long distance assimilatory process of vowels, either progressive or regressive. When used in this sense, the term vowel harmony is synonymous with the term metaphony.

In the second sense, vowel harmony refers only to progressive vowel harmony (beginning-to-end). For regressive harmony, the term umlaut is used. In this sense, metaphony is the general term while vowel harmony and umlaut are both sub-types of metaphony. The term umlaut is also used in a different sense to refer to a type of vowel gradation. This article will use "vowel harmony" for both progressive and regressive harmony.