This paper outlines Q theory, in which the traditional segment (consonant, vowel) is decomposed into a string of three ordered subsegments, or q, representing the onset, target, and offset of the segment. The postulation of subsegmental structure permits the representation of complex (contour) segments as well as subtle contrasts in segment-internal changes of state. Q Theory synthesizes insights from Autosegmental Phonology, Aperture Theory, and Articulatory Phonology in a representation that standard phonological constraints can refer to. Q theory is supported by arguments that subsegments act independently and need to be independently referenced by the phonological grammar. Embedded into Agreement by Correspondence Theory, Q theory permits the analysis of contour assimilation as well as contour formation, both in the tonal and segmental domains.