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. This book will be of particular interest to researchers in phonetics, phonology, and morphology, as well as to cognitive scientists interested in how the grammar may include constraints that emerge from the physical aspects of speech.