The Eocene-Oligocene transition in the eastern Gulf Coastal Plain of Alabama and Mississippi occurs within a sequence of marine clastic rocks making up the Jackson Group and the lower Vicksburg Group. The placement and the nature of the Eocene-Oligocene boundary remain controversial after more than 20 yr of detailed study. In Alabama, the Eocene-Oligocene boundary is placed within a condensed section at the contact between the Shubuta Member of the Yazoo Clay and the Red Bluff Clay. In eastern Mississippi, the Eocene-Oligocene boundary is recognized through the use of planktonic foraminifera and calcareous nannofossils as also occurring at the Yazoo–Red Bluff contact, but this surface is considered an unconformity representing a sequence boundary. In the Mossy Grove core of western Mississippi, planktonic foraminifera, in conjunction with radiometrically dated bentonites, place the Eocene-Oligocene boundary within the upper Yazoo Clay. There is a pronounced turnover in benthic foraminiferal species between the Yazoo Clay of the Jackson Group and the Red Bluff Clay of the Vicksburg Group. This turnover does not occur at the biostratigraphic Eocene-Oligocene boundary within the Yazoo Clay in western Mississippi. Rather, it occurs above the unconformity associated with the base of the Vicksburg Group throughout the Gulf Coastal Plain. This unconformity was produced by a fall in relative sea level, possibly associated with the oxygen isotope event Oi-1. In some sections in the eastern Gulf Coastal Plain (Little Stave Creek, Saint Stephens Quarry, and the Mossy Grove core), the Eocene-Oligocene boundary as defined at the Massignano global stratotype section and point can be recognized. In most sections in the region, however, such as those in the Chickasawhay River valley of Mississippi, subaerial erosion associated with the basal Vicksburg unconformity has removed the latest Eocene and earliest Oligocene (pre–Oi-1) rocks. This means that in most cases in the eastern Gulf Coastal Plain, the Eocene-Oligocene boundary is coincident with a lithostratigraphic and allostratigraphic surface associated with the Oi-1 event rather than the highest occurrence of Hantkenina. The Jackson Group—Vicksburg Group contact, with its associated turnovers in benthic foraminifera, is important in identifying the Eocene-Oligocene boundary in the eastern Gulf Coastal Plain. Work currently under way by Charles Betz at Ball State University on stable isotopes in both the Yazoo Clay and Red Bluff Clay will help to further characterize this boundary in the Gulf Coastal Plain and enhance understanding of this important transition.