The paper attempts to determine the motivating causes of palatal vowel harmony (PVH). Previously suggested causal explanations of PVH are critically evaluated, especially the progressive palatal assimilation view of the origin of PVH. Data on PVH restrictions from Finnish and Turkish are examined against a set of perceptually motivated working tendencies. It is shown that PVH and “labial harmony” are only special cases of a single, unitary type of restriction, statable acoustically in terms of the frequency of the second formant. A perceptual theory of the causes of PVH is proposed and general phonetic conditions for and against the development of PVH in a language are suggested. On the whole, the paper is an argument for a substance-based approach to phonology.