The eastern half of New York State is a dialectologically diverse region around which several dialect regions converge—the Inland North, New York City, Western New England, and Canada. These regions differ with respect to major parameters of North American English phonological variation; and therefore the interface between them is of interest because the location and structure of their boundaries can illuminate constraints on phonological changes and their geographic diffusion. In this dissertation, interviews with 119 speakers in New York State are conducted and phonetically analyzed in order to determine the dialect geography of this region in detail. The sampled area is divisible into several dialect regions. The Inland North fringe contains communities that were settled principally from southwestern New England; here the Northern Cities Shift (NCS) is present, but not as consistently as in the Inland North proper. In the core of the Hudson Valley, there is clear influence from New York City phonology. The Hudson Valley fringe, between the Hudson Valley core and the Inland North, exhibits some NCS features, but no raising of /æ/ higher than /e/; this is attributed to the effect of the nasal /æ/ system in blocking diffusion of full /æ/-raising. The North Country, in the northeastern corner of the state, is the only sampled region where the low back merger is well advanced, but the merger is in progress over the long term in the other regions except for the Hudson Valley core; this illustrates that the NCS does not effectively prevent merger. General theoretical inferences include the following: (potentially allophonic) segments, not phonemes, are the basic unit of chain shifting, and one allophone can prevent another from being moved into its phonetic space; the effect of diffusion of a phonemic merger from one region to another may merely be a slow trend in the recipient region toward merger; and isoglosses for lexically-specific features may correspond better to popular regional boundaries than do phonological isoglosses. Finally, a definition of dialect boundaries as obstacles to diffusion is introduced.