This article reviews the analysis of vowel harmony in Optimality Theory. Vowel harmony is a phenomenon in which the vowels in a word or another domain show systematic agreement for some property, such as rounding, backness, height, or ATR quality. Optimality Theory is a framework of generative linguistics in which grammars consist of a hierarchy of constraints on outputs. The treatment of various aspects of harmony systems are discussed, including what drives harmony, directionality and trigger control, opaque and transparent segments, dominant-recessive patterns, and variation. Data from Turkish, Igbo, Pulaar, Diola Fogny, Finnish, and Hungarian are discussed.