Marine strata of the Middle Devonian Hamilton Group in eastern New York State consist of a thick succession of synorogenic basinal to shoreface sediments. Initial basinal dark to black shales grade upward through increasingly coarse, sand-dominated facies. Beginning in Marcellus (early Givetian-age) strata in the Hudson Valley, these marine facies grade upward and laterally into terrestrial strata. This major transition occurs diachronously with higher strata undergoing transition to terrestrial facies progressively further westward. Time-rock/allostratigraphic relationships in these rocks have long remained poorly understood. This chapter examines older and recent observations and presents new interpretations and hypotheses on the correlation of these strata. The purposes of this work are to: 1) better constrain the position of the marine to terrestrial transition in upper the Marcellus subgroup strata; 2) better constrain the contact “interval” of the marine Mount Marion-Panther Mountain formations; 3) present current knowledge and hypotheses about the Panther Mountain Formation; 4) discuss the presence of two intervals of unique fauna, each consisting of an epibole of the brachiopod Schizophoria King, 1850 in southern Albany County and northeastern Greene County together with correlation of these intervals southward into Ulster County; 5) discuss basal strata of the upper Hamilton Cooperstown Formation through Schoharie to Greene counties; and finally, 6) present an overview of development of the Hamilton Group between Kingston, Ulster County and Port Jervis, Orange County, where New York, New Jersey, and Pennsylvania meet. Methods utilized include reviews of older reports, new findings, sequence stratigraphy, correlation of distinct marker units, some faunal assessment (e.g., Schizophoria fauna intervals and other faunal associations, including a coral-rich bed), and some lithologic indicators of relative depths (e.g., thin, decimeter-scale conglomerates). The synthesis and resulting interpretations presented in this chapter are more tentative than in some chapters; however, the authors attempt herein to lay the groundwork for future research.