This book examines the formal bases of postvelar harmony and its crosslinguistic variation. It is of interest especially to phonologists concerned with segmental harmony and its explanation within Optimality Theory. Postvelar harmony in two unrelated languages, Palestinian Arabic and St'át'imcets Salish, is examined in detail. The result is the first comprehensive clarification of postvelar phonology for either language. Two harmonies are distinguished: uvularisation harmony ('emphasis spread') and pharyngealisation (tongue-root-retraction) harmony. The distinction between these two in the Arabic and the Salish is supported by much instrumental phonetics data. The complex harmony properties are explained as the result of systematic interaction between Correspondence, Alignment and Grounded constraints. In the course of the investigation, the segmental inventories of both languages are clarified, and a careful understanding of the distinction between phonology and phonetics, and the use of phonetics in phonology, is applied.