This work elucidates the nature of the notion of Locality in phonology, describing the minimal conditions under which sounds assimilate to one another. The central thesis is that a sound can assimilate to another sound only if gestural contiguity is established between these two sounds. The argument supporting the central thesis of this book is unique in bringing evidence from articulatory dynamics, electromyography, and cross-linguistic sound patterns to converge on the same notion of locality in phonology. The main analytical focus is on phenomena that at first appear problematic for the central thesis but, in fact, turn out to provide striking confirmation of its correctness. In particular, attention is drawn to the kind of assimilation called "long distance," where the target and trigger of the assimilation process are not adjacent in the phonological string. Consonant Harmony, for example, is an apparent assimilation between the two consonants in a CVC sequence that seems to skip the vowel. A cross-linguistic investigation reveals a rather restricted typology of consonant harmony: the gestural parameters subject to assimilation are only those describing the mid-sagittal and cross-sectional shape of the tongue tip-blade. It turns out that these are precisely the consonantal gestural parameters that can propagate through the intervening vowel without a significant effect on the acoustic quality of that vowel. Hence, the specific properties of consonant harmony confirm the prediction of the central thesis that assimilation between the two consonants in a CVC sequence propagates through the vowel.